Ultra-Stable Lasers
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Ultra-Stable Lasers First Semester Report Fall Semester 2011
- Full report –
by Steven Dorlac Andy Wiersma Daxsamuel Chitechi Derrick Benallie
Prepared to partially fulfill the requirements for ECE401/ECE402
Department of Electrical and Computer Engineering Colorado State University Fort Collins, Colorado 80523
Project advisor: Prof. Randy Bartels
1 ABSTRACT
As technology advances computers are working faster and faster. With faster speeds, more accurate time keeping methods are needed. Optical clocks are one method being researched to replace atomic clocks in their role for accurate time keeping. This experiment is designed to investigate and improve the use of stable optical combs that will replace the current atomic clocks with more accurate time keeping. In this project we will build an ultra-stable external cavity diode laser, as well as analyzing electronics for temperature stabilization, low-noise diode drivers, and control electronics for feedback stabilization of temperature and laser frequency. The ultra-stable laser will improve the research on communications to remote synchronization. This semester we are focused mainly on the design and testing of the electronic components of the ultra-stable laser. The electronics will be composed of four different circuits, which are a low noise power supply circuit, thermal electronic control circuit, diode driver circuit, and a loop electronics circuit. For the spring semester of 2012 we are going to combine the four circuits with a laser design to create an ultra- stable laser for time keeping devices. Through analyzing different laser setups we were able to narrow down which ones applied the best to optically based clocks. We decided to base out laser off of the designs made by JILA of CU Boulder. There are many parts of their design that we found were still not fully applicable to our desired function so this paper will focus on the parts that are the most relevant.
2 TABLE OF CONTENTS Title……………………………………………………………………………………………..….i
Abstract………………………………………………………………………………………...….ii
Table of Contents…………………………………………………………………………………iii
I) Introduction………………………………………………………………………………..1
II) Optical Clockworks……………………………………………………………………….2
A) Optical Clock Intro………………………………………………………………..2
B) Doppler Broadening……………………………………………………………….3
C) Overview of Mode-locking and Frequency Combs……………………………….3
III) Laser Diodes………………………………………………………………………………4
A) Introduction………………………………………………………………………..5
B) Physics…………………………………………………………………………….5
C) Types………………………………………………………………………………7
C.i) Heterostructure Laser Diodes………………………………………..……………7
C.ii) Quantum Well Laser Diodes………………………………………………………8
C.iii) Distributed Feedback Lasers………………………………………………………8
C.iv) Vertical Cavity Surface-Emitting Laser Diode……………………………………9
C.v) External Cavity Diode Laser………………………………………………………9
D) Characteristics……………………………………………………………………10
D.i) Coherence……………………………………………………..…………………10
D.ii) Power…………………………………………………………….………………11
D.iii) Temperature Dependencies………………………………………………………11
IV) Mode Locking……………………………………………………………………………12
A) Mode-Locking……………………………………………………………………12
3 B) Types of mode-locking………………………………..…………………………13
B.i) Active……………………………………………………………………….……13
B.ii) Passive……………………………………………………………………………15
B.iii) Hybrid……………………………………………………………………………16
C) Applications of mode-locking a laser……………………………………………16
D) Issues with mode-locking…………………..……………………………………16
V) Frequency Combs……………………………………………..…………………………18
A) Introduction to Frequency Combs………………………….……………………18
B) Frequency Comb generation………………………………..……………………19
C) The Carrier-Envelope Offset………………………………..……………………20
C.i) Carrier-Envelope Offset Stabilization……………………………………………21
D) Noise in Frequency Combs………………………………………………………22
E) Applications……………………………………………………………...………23
VI) Fall 2011 Semester Plans/Accomplishments…….………………………………………24
A) Introduction………………………………………………………………………24
B) Current Controller Circuit…………………………..……………………………25
C) Thermoelectric Controller (TEC) circuit……………...…………………………28
D) Power Supply……………………………………………….……………………32
E) Control/Filter Loop Electronics…………………………….……………………36
VII) Spring 2012 Semester Plans……………………………………………...………………39
References……………………………………………………………………………..…………40
Bibliography……………………………………………………………………………..………42
Appendix A) Abbreviations……………………………………………………………...……A-1
Appendix B) Budget………………………………………………………………………..…B-1
4 LIST OF FIGURES
Figure 1-Schematic of Optical Clock………………………………………..……………………2
Figure 2- Laser Mode Structure……………………………………...……………………………2
Figure 3- Mode Locking Laser……………………………………………………………………3
Figure 4 –Dirac Function……………………………………………………….…………………4
Figure 5 – PN Junction……………………………………………………………………………5
Figure 6 Direct vs. Indirect Bandgap………………………………………………………...……6
Figure 7 – Heterostructure Diode…………………………………………………………………7
Figure 8 – Quantum Well Diode…………………………………………………………..………8
Figure 9 – VCSEL…………………………………………………………………………...……9
Figure 10 – ECDL Configurations……………………………………………………….………10
Figure 11: Generation of a pulse train from optical oscillations interfering with each other....…12
Figure 12: mode locked spectrum and a spectrum that is not properly locked……………..……13
Figure 13: Setup of an actively mode-locked laser ………………………………………..……14
Figure 14: Setup of a passively mode-locked laser………………………………………...……15
Figure 15: frequency comb of a mode-locked laser……………………………………...………18
Figure 16: Electric field of laser pulses with a 5fs duration and variable CEO phase……..……20
Figure 17: Principle of the common f-to-2f self-referencing scheme…………………...……….21
Figure 18: Feedback portion from Jila Current Controller Schematic………………….……….25
Figure 19: Output to Laser Diode from Jila Current Controller Schematic……………..………26
Figure 20: Current Line from Jila Current Controller Schematic………………………………..26
Figure 21: Supply to Current Line from Jila Current Controller Schematic…………….………27
Figure 22: JILA Temperature controller circuit…………………………………………...……..29
5 Figure 23: Low power, low cost 2.5V Reference……………………………..…………………30
Figure 24: Low power, low cost 2.5V Reference top view…………………………..…………30
Figure 25: Gain stage of the temperature controller…………………………………..…………31
Figure 26: Peltier cooling and heating stage………………………………………..……………32
Figure 27-Pulsed Power Supply………………………………………………….………………
34
Figure 28-Typical Input vs. Output Voltages…………………………………..………………..35
Figure 29-DC to HV DC Converter Table (G03)……………………………….……………….35
Figure 30 – Controller Block Diagram.…………………………………………………………36
Figure 31 – JILA Controller…………………………………………..…………………………37
Figure 32 – JILA Controller………………………………………..……………………………38
6 I. Introduction:
This experiment is designed to investigate and improve the use of stable optical combs that will replace the current atomic clocks with more accurate time keeping. In this project we will build ultra-stable external cavity diode laser, designing electronics for temperature stabilization, low- noise diode drivers, and control electronics for feedback stabilization of temperature and laser frequency. The ultra-stable laser will improve the research on communications to remote synchronization. For the fall semester we focused mainly on the design, build and testing of the electronic components of the ultra-stable laser. The electronics will be composed of four different circuits, which are a low noise power supply circuit, thermal electronic control circuit, diode driver circuit, and a loop electronics circuit. For the spring semester of 2012 we are going to combine the four circuits with a laser design to create an ultra-stable laser for time keeping devices.
7 II. Optical Clockworks
A. Optical Clock Intro:
An optical clock (figure 1) is a clock that is based on fundamental physics of an electron in an atom. When these atoms are subject to an electromagnetic field, they absorb energy and jump to a higher energy state. As these electrons re-emit their energy, they drop back down to lower states. If a feedback loop consists of electrons that continually fluctuate between two levels, you can construct an ultrafast and a precise subatomic pendulum.
An optical clock can offer an extremely high frequency precision and stability which are kept in an optical trap. When you apply laser cooling, which is limiting the random motion of the dissipative light forces, you reduce the collisions and the temperature of these particles to suppress Doppler broadening.
B. Doppler Broadening:
Doppler broadening is the broadening of spectral lines due to the Doppler Effect. This is caused by a disturbance of velocities in the atoms. The atoms of reemitting energies have different velocities that result in different Doppler
8 shifts that cause line broadening. Doppler broadening can have severe constraints on spectroscopic measurements.
is the Doppler broadening, which depends on the frequency of the spectral line, the mass of the emitting particles, and their temperature. By reducing the temperature by laser cooling, it decreases atomic collisions; therefore you can have more precise experimental outcomes.
Optical Clockworks are frequency chains that contain complicated combination of many nonlinear stages. Each of these stages has some frequency that contains multiples of that same frequency. Not only are these frequency chains difficult to make, but we’re limited to certain isolated optical frequencies. With the use of frequency combs that are produced from mode- locked lasers it makes it simpler and more versatile to produce optical clockworks.
C. Overview of Mode-locking and Frequency Combs:
A mode locked laser (figure 6) is a technique used in optics that produces pulses of light in a very short duration. This technique is based on an induced fixed phase between the modes of the laser’s resonant cavity. This is what makes the laser become mode-locked. When the laser is mode-locked, it produces a train of pulses that can be as short as a few femtoseconds. These lasers are able to produce frequency combs, which are determined by the pulse repetition frequency and the carrier envelope offset frequency. A certain integer multiple of the pulse repetition frequency and a beat note frequency can be measured and processed with fast electronics. The optical cavity of the laser determines the laser’s emission frequency. Basically by facing two flat plane mirrors toward each other, the light of the laser oscillates back and forth between the mirrors and produces a gain. The light begins to destructively and constructively interfere which creates modes between the mirrors. These are the longitudinal modes of the cavity. These modes have a narrow range of frequencies where it operates, which is the 9 bandwidth. For the above figure, SA is saturable absorber mirror. This means that the reflectance of this mirror is 100% and that the incoming signal will be totally reflected. OC is a coupler mirror that has a reflectance less than 100%, so minimum amount of the signal will propagate through the mirror but the majority of the signal will be reflected back into the cavity.
When the mode lock lasers generate the optical pulses, as seen through the spectrum, you will see a series
Dirac delta functions (figure 4). These functions are produced by the oscillations of the light reflecting back and forth between the mirrors of the cavity. The separation of these Dirac functions are separated by the time it takes the light to complete one full cycle.
III. Laser Diodes A. Introduction:
Laser diodes possess many features that give them a wide range of applications from simple low power laser pointers to very high powered laser diode arrays found in laboratories and manufacturing. They are available in a wide arrange of wavelengths, power ratings and geometries. Additionally, they are compact and often possess a high electrical to optical efficiency lending themselves to applications with space and power restrictions. What follows is a brief description about the theory of operation behind laser diodes in addition to some of the more common types available.
10 B. Physics:
Lasers Diodes form a subset of the diode family. Like all diodes, their simplest form is composed of two differently doped regions within the chip, a p-doped region and an n-doped region. The junction of these two regions creates what is known as a depletion region. This depletion region is characterized by an absence of charge carriers caused by the combination of
“holes” in the p-region and electrons in the n- region. When an electrical potential is applied to the diode in the forward direction, electrons and holes are injected into the depletion region from the n and p regions respectively allowing current to flow. When the depletion region has both holes and electrons they can recombine which results in the release of energy either in the form of light (photons) or heat (phonons). A graphical representation of this is seen with a band-gap diagram pictured below.
The main difference between a standard diode and a light emitting diode (LED) lies in the alignment of the valance band and conduction band in the band-gap diagram. This alignment allows electrons to transition to the valence band directly releasing the energy as light (photons).
In a normal diode the bands are misaligned causing the release of energy in the form of heat
(phonons). Laser Diodes are a specialized form of an LED. The ends of a laser diode are polished or cleaved in such a way that the ends form an optical resonance cavity. Additionally, a laser diode has an increase in the amount of carriers present due to a modification of both current and geometry creating an abundance of electrons and holes with the potential to combine. With this increase in carrier population it now becomes possible for a passing photon released from an electron and hole combination to trigger another combination without being absorbed. This combined with the resonance cavity allow for the generation of more photons with matching 11 phase. A portion of this coherent light is emitted from the diode chip since the ends are not completely reflective. This light is what comprises the laser produce by the laser diode.
C. Types
The physics discussion above was
limited to a single p-n junction laser
diode due the need to simplify its explanation. This type of laser diode is known as a homojunction and is subject to limitations and inefficiency issues that are not present in more complex laser diode types. Most prevalent was the high current density that required the diode to operate at very low temperatures. i. Heterostructure Laser Diodes:
Hererostructure laser diodes are constructed of multiple n and p type materials. The double heterostructure (DH) diode laser is one of the most common types of laser diodes in use today.
Unlike their homojunction counterpart they have a lower current density and are operable at room temperatures. This type of laser diode also sees an increase in efficiency (see image below).
"- Photon confinement in the GaAs active region
due to the larger index of refraction of GaAs (n
= 3.6) compared to the p- and n- cladding layers
(n = 3.4).
-Carrier confinement in the GaAs active region
due to the smaller band gap (Eg ≈ 1.5 eV)of the
GaAs compared to the p- and n- cladding layers (Eg ≈ 1.8eV). 12 Reduction in photon absorption arising from the differences in band gap of the active
and cladding layers. Only photons created with energy equal to or greater than the
larger band gap cladding layer are absorbed. This results in only minor absorption at
the blue tail of the emission profile."[10]
ii. Quantum Well Laser Diodes (QWLD):
Quantum Well laser diodes are a class of DH diode in which the active middle region is reduced in thickness.
These laser diodes feature a current density 4-5 times lower than DH diodes and possess a higher differential gain that is less susceptible to thermal variations[10]. QW laser diodes can also be found in what is known as a
Multiple Quantum Well (MQW) and Strained Quantum
Well (SQW) configurations. MQW diodes possess a series of alternating narrow and wide band gap materials. SQW diodes are QW diodes that have a lattice structure mismatch. iii. Distributed Feedback Lasers (DFB):
These laser diodes feature an internal diffraction grating that is etched close to the active region of the diode. This grating acts as a filter and reflects a predetermined wavelength back into the gain region. This removes the requirement that the ends or facets of the diode be polished to act like mirrors. The main feature of this type of laser diode is its stable output frequency determined by the grating.
13 iv. Vertical Cavity Surface-Emitting Laser Diode (VCSELs):
VCSEL diodes possess a vertical optical cavity as opposed to
the laterally oriented cavity of the previous laser diodes. In
this orientation light travels parallel to the direction of current.
The active layers are typically composed of multiple SQW
layers between quarter-wave Bragg reflectors. Due to the
short gain length the output power of these diodes is typically
less than other laser diodes. However, this configuration allows for a greater density of emitters.
v. External Cavity Diode Laser (ECDL):
In an ECDL the laser diode has one facet coated with an anti-reflection coating. The output from this facet is then directed through a collimating lens to an external mirror that completes the optical cavity of the laser. This type of laser is also often known as a tunable diode laser since the output wavelength can be adjusted via external mirrors and diffraction gratings. There are two configuration types that are common with ECDLs. These are the Littrow configuration and the Littman-Metcalf configuration. In the Littrow configuration the external cavity consists of the collimating lens and a diffraction grating with the laser output coming directly from the grating. Adjusting the wavelength is accomplished by rotating the diffraction grating. The disadvantage of this is that the output direction changes as the diffraction grating is adjusted. In 14 the Littman-Metcalf configuration a mirror is added to the external cavity in addition to the collimating lens and the diffraction grating. Wavelength adjustment is carried out by adjusting the mirror and the output comes from the stationary diffraction grating. This alleviates the problem with output direction that characterized the Littrow configuration but comes at the cost of reduce power due to the loss of the zero-order reflection by the tuning mirror.
Figure 10: ECDL Configurations [6] D. Characteristics:
Laser diodes come in a wide variety of wavelength and power configurations. Combine this with their size and efficiency and offer a very wide range of application. They can, however, be difficult to control and the light they emit possesses characteristics that are often undesirable. i. Coherence:
Due to the small cavity length and facet geometry a typical laser diode emits an elliptical cone of light. This divergence is often measured with the full width-half maximum (FWHM) light power in the perpendicular and parallel axis with regard to the active region of the laser diode.
Typical values are in the area of 30 in the parallel axis and 10 in the perpendicular axis [6]. To correct this, a collimating lens is typically employed to collimate the beam and reduce divergence. Laser diodes also suffer from a slight astigmatism as a result of the unequal
15 divergence of the beam. The wide divergence places the source of the beam much closer to the facet than the narrower divergence.
ii. Power:
Laser diodes come in a wide array of power outputs depending on the application. Lower power applications such as CD players and laser pointers typically have a laser diode less than 1 mW to
5 mW. Laser printers may can use laser diodes anywhere from 5 mW to over 50 mW depending on the speed and resolution of the printer[13]. HD DVD and Blu-Ray burners can contain laser diodes from 100 mW to 500 mW. The laser diodes with power ratings greater than 500 mW are typically found in laboratory and industrial settings and can reach 100s of Watts. iii. Temperature Dependencies:
Laser diodes are susceptible to temperature changes. An increase of 1 K will typically increase the wavelength by 0.3 nm [6]. This causes the output power to drop assuming the diode is being operated with a constant current source. Additionally, high operating temperatures increase the likelihood of failure. Due to the high current densities present in laser diodes there is a concern about thermal runaway. To alleviate this and to maintain a steady output wavelength and temperature, some form of cooling is employed which can be either passive or active. For precision applications active cooling is utilized with a temperature controller to maintain the laser diode at a set temperature.
16 IV. Mode Locking
A. Mode-Locking:
Mode-locking is a technique used to generated light pulses with very small duration. Using mode locking methods the duration of the pulses can be reduced to durations in the picosecond
(10-12) to femtosecond (10-15) range [14]. Sometimes mode-locking is also referred to as phase- locking. In regular lasers the pulse is restricted by the cavity size of the laser. By using the mode- locking technique the pulse size can be reduced by many orders of magnitude. The figure below shows multiple oscillations of a wave in a cavity shown in blue. The red is a summation of the oscillations. Since the signal is mode-locked the summation is a train of small pulses.
If the oscillations are not able to resonate correctly the output becomes random pulses. Figure 6 is an example of this. The blue line illustrates a pulse train that has successfully been mode- locked. The red waveform is the outcome of an incorrectly executed attempt at mode-locking.
17 Since the wave did not resonate correctly the strong and weak intensity parts of the wave build on each other and create a signal that does repeat but does not pulse.
Origin of the term mode-locking:
Although it may make more sense to look at the mode-locking mechanisms in the time domain,
the term originates from the frequency domain.
B. Types of mode-locking: i. Active:
Active Mode locking is done by having a modulator somewhere in the cavity of the laser. As the pulse travels back and forth through the laser cavity the modulator resonates losses that it creates into the wave. In order to effectively achieve a mode-lock the modulator has to work in synch with the pulse train. The modulator frequency is a direct function of the resonator cavity because the resonance frequency depends directly on the amount of time it takes for the pulses to resonate in the cavity. Active mode-locking can be done with pumping the laser on and off or with 18 acousto-optic, electro-optic, Mach-Zander integrated optic, or semiconductor electro-absorption modulators. These methods of mode locking use the modulator to put both constructive and destructive interference into the wave. Active modulation can be done by either implementing loss modulation or phase modulation. Since it is synced with the period of the wave the constructive and destructive parts create pulses from the wave. This method is good for generating a clean mode locked signal as long as the modulation is actively synced because the mode locking is done by a signal not dependent on the wave. The downside to this type of mode-locking is that the speed is restricted by the frequency of the external signal. Below is an illustration of how the active type mode-locking works.
Figure 13: An illustration of how an actively mode-locked laser is set up
ii. Passive:
Passive mode-locking is done without the use of an external signal such as the methods used in active mode locking. Instead, passive mode-locking systems use a saturable absorber to mode- lock the system. The saturable absorber has to be set so it effectively blocks parts of the wave
19 with a weak intensity while the parts of the wave with a stronger intensity are allowed to pass through. There are two typical types of saturable absorbers. The two types are fast and slow saturable absorbers. The main differences in these two types of saturable absorbers are the speed at which they become saturated and the gain affect they have on the light. These both cause the strong intensity parts of the wave to constructively build on themselves while the weaker parts continually get absorbed though. By imposing this constructive and destructive pattern it creates a pulse train. Unlike the active mode-locking, passive mode-locking is not restricted in speed by any external variables. This causes the pulse sizes to be much smaller than the in its active counterpart. The downside of passive mode-locking is any noise in strong intensity parts of the wave is amplified.
Figure 14: An Illustration of how a passively mode-locked laser is set up [3]
iii. Hybrid:
Hybrid mode-locking is a way of taking advantage of both the active and passive mode-locking systems. The passive part of the mode-locking can act faster and the active mode-locking can still be used to filter out repetitive spurs created by the laser. The down-side of this type of
20 system is that it is more costly because you have to implement both types of mode-locking.
There are also more losses because the light has to pass through mediums for both the active and passive parts. The overall outcome of this type of system is a output that can be very fast though with not too much noise from spurs.
C. Applications of mode-locking a laser:
The purpose of a mode-locked laser is to create an optical frequency comb. Depending on how a laser is mode-locked the pulse widths of the frequency comb can vary. The shorter the pulse, the more accuracy you can get in frequency comb applications such as time keeping or in navigational systems. This pushes for mode-locking techniques that create pulses that are as short as possible. Current pulses can be as small as hundreds of femtoseconds.
D. Issues with mode-locking:
Since mode-locking is based off of both constructive and destructive interference noise is a big issue. Any time you deal with the constructive interference any noises that are being transferred into the system have to be taken into account. This is because if the noises are in phase with the constructive interference they will be amplified and could potentially grow large enough to impact the actual signal. Another issue is slight changes in the timing such as timing jitter. Even though these changes are very small, the pulse sizes of a mode locked laser can be very small as well. This means that the slight differences could be significant when you look at them on the same scale as the pulses created by a mode-locked laser.
21 V. Frequency Combs
A. Introduction to Frequency Combs:
A frequency comb is the graphic representation of the spectrum of a mode locked laser.
According to the encyclopedia of laser physics and technology, an optical frequency comb is an optical spectrum which consists of equidistant lines.
Figure 15: frequency comb of a mode-locked laser, with an exaggerated mode spacing of 25Hz [6] The frequency comb can be used as an optical ruler whereby, if the comb frequencies are known, the frequency comb can be used to measure unknown frequencies by measuring the beat notes, which reveals the difference between the unknown frequency and the comb frequency. One thing to keep in mind is that when it comes to performing such measurements in a wide frequency range, a large overall bandwidth of the frequency is needed.[6]
B. Frequency Comb generation:
22 According to research attempts to produce broadband frequency combs were based on strongly driven electro-optic modulators, which could impose dozens of sidebands on a single-frequency input beam from a single-frequency continuous-wave laser. It was discovered that this process could be made more efficient by placing the modulator in a resonant cavity, particularly when the intra-cavity dispersion was minimized. Such devices acquired an increasing similarity to mode- locked lasers for ultra-short pulse generation. This led to the realization that a femtosecond mode-locked laser could actually be used very well when it came to generating very broadband frequency combs. The optical spectrum of a periodic pulse train, as generated in a mode-locked laser, consists of discrete lines with an exactly constant spacing which equals the pulse repetition frequency. The main thing to remember is that the generation of a frequency comb requires that the periodicity applies not only to the pulse envelopes, but to the whole electric field of the pulses, including their optical phase, apart from a constant phase slip. In other words coherence between the pulses is required. [6]
Mode locked lasers produce a series of optical pulses separated in time by the round-trip time of the laser cavity. The spectrum of such a pulse train is a series of Dirac delta functions (in the context of signal processing it is often referred to as the unit impulse function) separated by the repetition rate of the laser. This series of sharp spectral lines is called a frequency comb or a frequency Dirac comb. A purely electronic device, that generates a series of pulses, also generates a frequency comb.
C. The Carrier-Envelope Offset:
According to the research, if a pulse train were perfectly periodic, all the frequencies of the lines in the spectrum would be integer multiples (harmonics) of the pulse repetition rate. Most cases, 23 however, intracavity dispersion and nonlinearities cause a slip of the carrier-envelope offset
(CEO) from pulse to pulse, that is, the oscillations of the electric field are constantly shifted with
respect to the pulse envelope below.
If the change in the carrier-envelope
offset per resonator round trip is constant
(denoted Δφceo), all optical line frequencies
can be written as [6]
Where j is an integer index and frep is the pulse repetition rate and the CEO frequency, which can be between 0 and frep.
[6]
Note if the two parameters frep and Vceo are known, all the frequencies of the comb are also known. In that case, any optical frequency within the range of the frequency comb can be determined by recording a beat note between the unknown frequency and the comb. The lowest beat frequency is the distance from the unknown frequency to the nearest line of the comb. An approximate frequency measurement can be used to determine from which line the detected beat originates. It is then possible to figure out whether the unknown frequency is above or below the comb line frequency. The pulse repetition rate frep is easily measured with a fast photodiode, whereas the measurement of νceo is significantly more difficult. It can be detected e.g. via an interferometric f−2f self-referencing scheme [4, 5], where one uses a beat note between the frequency-doubled lower-frequency end of the comb spectrum with the higher-frequency end, if the spectrum covered an optical octave. [6]
24 Figure 17: Principle of the common f-to-2f self-referencing scheme [6] Such broad spectra can be achieved, with super-continuum generation in photonic crystal fibers, if the laser output itself does not have sufficient bandwidth. Note it is possible to generate octave-spanning spectra directly with titanium-sapphire lasers. [6]
i. Carrier-Envelope Offset Stabilization:
According to research, for some applications, the CEO frequency is stabilized with a feedback system using an error signal from an f-2f interferometer. The CEO frequency may be fixed at 0 or at any given value, or at a certain fraction of the pulse repetition rate. The weaker form of
CEO stabilization means that the excursions of the CEO frequency are limited, but the CEO phase may still drift away. The stronger form is real CEO phase stabilization [8, 9, 12], where the
CEO phase either stays fixed or advances from pulse to pulse by a predictable value. Here the uncertainty in the CEO phase should be well below 1 rad. Most important thing to note is that even with a stabilization based on feedback from the error signal obtained with an f-2f interferometer may be unable to prevent drifts of the CEO phase.
A totally different way of obtaining a CEO-stabilized frequency comb is to do difference frequency generation of different parts of the comb spectrum. In that case, the nonlinear mixing
25 product has a zero CEO frequency. When CEO-stabilized pulses are sent through a high- gain amplifier, the CEO phase stability may be lost in the amplifier. However, it is possible to construct amplifiers which will preserve the CEO phase. [6]
D. Noise in Frequency Combs:
Different noise sources, such as mirror vibrations, thermal drifts, pump intensity noise and quantum noise, cause different and partly correlated combinations of noise of the pulse repetition rate and the carrier–envelope offset frequency. Also there is some level of noise in all lines of a frequency comb which is not correlated. For example, resonator length changes have hardly any impact on the CEO frequency but influence the pulse repetition rate, which would be the line spacing. From research an important theoretical finding is that the quantum-limited CEO noise of mode-locked lasers with relatively long pulses is larger than from a laser with few-cycle pulses but otherwise similar parameters, meaning that there is substantial experimental evidence for significantly stronger noise from fiber lasers as compared with titanium–sapphire lasers, which generate shorter pulses. [6]
E. Applications:
Frequency combs have dramatically simplified and improved the accuracy of the frequency metrology. The frequency combs are making it possible to build optical atomic clocks, which are expected to be as much as 100 times more accurate than today’s best time-keeping systems.
Better clocks will lead to studies of the stability of the constants of nature over time, and enable improved technology for advanced communications and precision navigation systems, such as the next generation global positioning systems.
Frequency combs can also be used for the measurement of absolute optical frequencies.
This means that optical frequencies are related to the microwave frequency, for example, from a cesium clock. In other words, a frequency comb can serve as an optical clockwork. [6] 26 Frequency combs can also be used to measure ratios of optical frequencies with extremely high precision, which is not even limited by laser noise. [6]
According to research applications of frequency combs are possible in high-precision spectroscopy, optical sensing, distance measurements, laser noise characterization, telecommunications, and in the fundamentals of physics. [6]
27 VI. Fall 2011 Semester Plans/Accomplishments
A. Introduction:
For the fall Prof. Bartels explained to us that we were going to focus on the design, build and testing of the electronic control components of an ultra-stable diode laser. The electronics are composed of four different circuits; low noise power supply circuit, thermal electronic control circuit, diode driver circuit, and a control/filter loop electronics circuit. Each person of our team was assigned a circuit that they would research and determine the most applicable circuit. Prof.
Bartels gave us a couple of links that had different circuits that might fit our needs. After extensive research, we decided to use the JILA circuits, since they were more advanced.
The main goal of the semester was to study the JILA circuits, implement and simulate them within Cadence to further our understanding on each of the circuits assigned to us. With the completed simulations we would then create and send the PCB designs to be fabricated. Once we received the PCB boards and assembled the circuits we would begin real testing with the external cavity diode laser. After spending some time attempting to implement the circuits within
Cadence we concluded that Cadence was the incorrect tool for this task. This was largely due to the lack of circuit components available to us within Cadence. We would have been required to build the majority of the components from the transistor level. From there we opted to use
OrCAD capture which contained the majority of the components. In addition it would also allow us to create PCB designs of the JILA circuits via OrCAD PCB editor. Also, to add to the accomplishments of this semester we ordered parts for each of the circuits. Once the parts arrive we will prototype the circuits on breadboards and test them to ensure that the circuits function appropriately before sending the PCB designs to be fabricated.
B. Current Controller Circuit:
28 The current controller circuit is used to apply a steady current to the diode laser. To describe the current controller I will try to break it down into smaller sections. Each section of the current controller sections has a different operation but ultimately is trying to reduce variations in the current. This is done by applying filters and regulators at different points as well as using feedback to stabilize changes. Although the current controller we are building has more sections, the other sections are predominantly for external monitoring and do not do too much for the operation for the circuit as a whole.
The first section is the input to the current controller from the servo. This section shown in
Current Controller Figure 1 is made up of two op amps and some resistors and capacitors. The purpose of the capacitors is to short AC variations from the power supplies to ground so they do not get fed into the amps. The resistors in this section are acting as feedback loops so that the op-amps can compensate for changes coming from the servo.
The output from this section is then fed to Figure 19 as a current that can resist changes sensed in the servo.
The second section is the output from the current controller to the diode. This section shown as
Figure 19 is made up of a switch, a diode. The switch on the right is a switch that can turn off
29 the current to the diode by shorting it to ground. The diode is used as a rectifier so the current to the diode is always going the same direction.
The next figure is shown as Figure 20. As it shows it is connected to all the other circuit parts I have discussed as well as to figure 21.
The purpose of this part of the circuit is to supply the current needed the laser diode. The two bottom three inputs are the main feeds into the circuit. The two inductors shown are used to reduce the amount of noise being transmitted into the laser diode.
Since accuracy is very important for the use of an optical clock the minimum amount of possible noise in the system is important. The
IRFF9220s are pnp transistor are used to make sure that there is no current flowing the wrong way through the circuit. The Last components of this circuit are the inductor and resistor in parallel used to resist alternating current from flowing through the line.
The last circuit that explains how the Current Controller operates is shown in Current Controller Figure 21. This circuit is used to supply the power to the current line that is shown as Figure 20. The key component of this circuit to take note of is the LM399H towards the left side of the circuit. The purpose of this is to regulate the voltage going to the rest of the circuit components. This component varies vary little even with temperature changes.
30 31 C. Thermoelectric Controller (TEC) circuit:
The purpose of a temperature control system on an external cavity diode laser (ECDL) is to maintain a constant temperature within the device. The two types of devices used to control the temperature of the ECDL or other temperature sensitive devices are the thermoelectric (peltier cooler) and a resistive heater. The temperature controller uses a current or voltage source to drive power through the peltier cooler and the heater based on feedback from a temperature sensor.
The temperature sensor in this case would be the thermistor and the thermocouple. The thermistor is able to achieve a temperature stability of 0.01oC to 0.001oC whereas the thermocouple can achieve a temperature stability of 1oC. Note the design of the system dictates the stability. [9]
I was assigned the TEC circuit, whereby I had to research on it using the links Dr.Randy
Bartels sent me and any other links I could find. I narrowed it down to the JILA circuits which had more potential than any other circuits that I researched on. The JILA circuits were advanced and easy to simulate using any type of software. Below is the JILA TEC circuit.
32 Figure 22: JILA Temperature controller circuit [1]
33 After extensive research on the circuit I found out that there are three main components to the entire circuit. The first component would be the low power low cost 2.5V reference (AD680).
The AD680 is a bandgap voltage reference which provides a fixed 2.5V output from inputs between 4.5V and 36V. The AD680 bandgap reference operates on a very low quiescent current which rivals that of many two-terminal references, hence making it ideal for use in power
sensitive applications. The AD680 noise is low, roughly
8µVP-P from 0.1Hz to 10Hz. The spectral density is also
low [5]. The diagrams to the left illustrate the AD680.
TP denotes factory test point and NC denotes no
connection.
The second key component to my TEC circuit was the gain
stage which consisted of two Quad picoampere input
current bipolar Op Amps (AD704). According to research the AD704 is a
quad, low power bipolar op amp that has the low input bias current of a BiFET
amplifier but which offers a significantly lower IB drift over temperature. Its features are high DC precision, low noise (0.5µVP-P , 0.1Hz to 10Hz) and low power [7]. The gain stage also contains a differentiator and an integrator. The diagram below illustrates the gain stage of the TEC.
34 Figure 25: Gain stage of the temperature controller [1] The third key component is the peltier cooling and heating stage. When a negative gain is obtained from the gain stage, the gain sign jumper connects to the 2&7 and 4&5 which enable the peltier cooling and heating stage to connect to the peltier cooler hence cooling the system.
When a positive gain is achieved from the gain stage, the gain sign jumper connects 1&8 and
3&6which enable the peltier cooling and heating stage to connect to the heater hence heating up the system. The peltier cooling and heating stage consist of a quad picoampere input current 35 bipolar Op Amp, a NPN epitaxial Darlington transistor [8] and a silicon PNP Darlington power transistor [2], which are useful in medium power linear switching applications. The diagram below illustrates the peltier cooling and heating stage.
Figure 26: Peltier cooling and heating stage [1] D. Power Supply:
36 In the beginning stages of our project, we decided to build all the circuits for the ultra-stable laser. Starting with the Power Supply Unit, it was essential to achieve a very low frequency noise since this is the initial device that will power our entire project. After some thorough research and investigation, we concluded that it would be easier, less expensive, and less time consuming to use the Power Supply Units that are already placed in the Electrical Engineering labs. These devices are already designed to have a low noise frequency; it was easier and less of a headache than trying to achieve these low frequency parameters. The margin of error was greatly reduced by using the Power Supply Unit from the labs to power the Thermoelectric
Control, Piezoelectric Transducer, and Current Controller.
The circuit I researched was the Piezoelectric Transducer (PZT) Pulsed Power Supply. This would create a high voltage power source that would power the PZT driver. This PZT driver has a PZT disk, which houses a mounded mirror that completes the External Cavity Diode Laser
(ECDL). The PZT Power Supply operates the PZT Driver. Once the voltage is varied in to PZT driver, the diode laser cavity also varies in length. Once the length is varied, the optical frequency signal will vary as well. The next page shows the schematic of the PZT Pulse Power
Supply. This circuit is symmetrical. The top portion creates a positive High Voltage and the lower part inversely creates a negative HV. The positive and negative terminals on the left of the schematic would be connected to the Power Supply respectively. 37 Figure 27: Pulsed Power Supply [1] The component that is circled in red is the EMCO G-Series 03, this remarkable device is able to take in an input voltage and then step up a high output voltage.
38 Figure 28: Typical Input vs. Output Voltages [15]
Figure 29: DC to HV DC Converter Table (G03) [15]
39 The G3 Series provides an output proportional to the input. As you can see from the Input vs.
Output Voltages plot, it is a linear plot. This component converts an initial DC voltage to a high voltage DC output with low noise. The initial voltage coming in from the Power Supply will provide the initial voltage for the G03and then will kick out a high voltage that will attenuate to the branch that is circled in green. The components circled in blue are power supplies, which power the circuits that are circled in orange. The components that are circled in orange are the switches of the circuit. As these switches toggle off and on, they create a very short pulse of high voltages then travel to the branch circled in purple. This branch is what powers the PZT driver and varies the cavity length, which the outputting diode laser will have varying frequencies.
E. Control/Filter Loop Electronics
The Control/Filter electronics is responsible for adding a feedback control element to the ECDL to help stabilize the output wavelength. It is often used in conjunction with other electronics depending how the laser diode is to be locked. In our application we need only mode lock the
ECDL so the electronics for peak locking and side locking are unnecessary. With this being the case, the controller is typically fed an input signal from the output of the ECDL via a photodetector/photodiode. Based on error and changes present within the input signal the controller sends an output to adjust the wavelength to its desired value. This output signal is most often used to control the position of the mirror or gratings within the ECDL depending on whether it is a Littrow or Littman-Metcaf configuration. This is accomplished by changing the voltage across a piezoelectric disc mounted on the back of the mirror or grating.
The circuit we studied and will attempt to implement is an analog PI control/filter designed by JILA of CU Boulder, CO. In our analysis we broke the circuit into three 40 components: a Mode Selection circuit, the PI circuit and a Input Monitor circuit. A block
diagram of the components and
their connections is shown in the
picture to the right.
The Mode Selection circuit
allows the user to select how the
controller operates depending on
where the controller is placed in
the system and the desired
operation. It features a rotary
switch that sets one of a set of four transistor switches present within the PI circuitry.
The Input Monitor component of the controller is simply a circuit that allows the input signal to be fed to an outside monitor. The final circuit is the PI control circuitry. This circuit is implemented with high grade opamps often seen in audio amplifiers with an emphasis on noise suppression and signal quality. Capacitors are also used within the circuit to filter out noise that might be present within the circuit power supply. There are three rotary switches that allow the user to set PI corner frequency, gain and -9dB corner frequency. In addition to the three rotary switches there is a toggle switch to select the sign of the gain and a trim potentiometer to fine adjust the gain. The final feature is another potentiometer to adjust output impedance.
41 Figure 32: JILA Controller [1]
Spring 2012 Future Plans
For the next semester we are going to build the ECDL by assembling the remaining needed components. Some of these components will need to be designed such as the external cavity. 42 Other components including the diode housing, piezoelectric discs, mirrors, gratings will probably be purchased once we determine the required specifications. We will have to research the schematic diagram for a piezoelectric (PZT) driver that will best fit our design. The PZT
Pulse Power Supply will supply the high voltage that the PZT driver requires. Once the driver has been built and tested it will be used to supply the voltage to the PZT discs which are mounted to the back of the cavity mirror. This will allow us to control the emitted wavelength by changing the optical resonance cavity. When the length varies, the laser diodes’ optical signal frequency output will vary as well.
After we have assembled the ECDL system, including the control circuits from this semester, we will begin to calibrate our control electronics and the system as a whole to achieve a stable wavelength output. With the Diode Laser (LD) system stable we will then move on to mode locking the laser in an attempt to get a stable optical comb. Time allowing we would then like to move on to setting up an optical clock with our mode-locked ECDL.
43 References
[1] JILA NIST-CU. (2008, Jun 17) [Online].Available http://jila.colorado.edu/~frahm/ce/circuit/. [2] Fairchild. ( 2008, October). “TIP125/TIP126/TIP127.” PNP Epitaxial Darlington Transistor. [Online]. Available: http://www.fairchildsemi.com/ds/TI/TIP126.pdf [Dec. 9, 2011] [3]Meiser, Niels. Mode Locking. Laser-Physics, KTH. [Online]. Available: http://www.laserphysics.kth.se/courses/laser_physics_siegman/lectures/lecture15-mode- locking.pdf [4] Wavelength Electronics. ( 2008). “ Temperature controllers.” High precision, Ultra stable Temperature Controllers and laser diode drivers. [Online]. Available: http://www.teamwavelength.com/info/temperaturecontrollers.php [Dec. 9, 2011] [5] Analog Devices. (2005). “ Low power, low cost 2.5V Reference.” ANALOG DEVICES. [Online]. Available: http://www.if.pw.edu.pl/~alice/AD680.pdf [Dec. 9, 2011] [6] Paschotta, Rüdiger. (24, Jul. 2011) Encyclopedia of Laser Physics and Technology. [Online]. Available: http://www.rp-photonics.com/ [7] Analog Devices. (2001-2010). “Picoampere input current Quad Bipolar Op Amp.” ANALOG DEVICES. [Online]. Available: http://www.analog.com/static/imported- files/data_sheets/AD704.pdf [Dec. 9, 2011] [8] Fairchild. ( 2008, October). “TIP120/TIP121/TIP122.” NPN Epitaxial Darlington Transistor. [Online]. Available: http://www.fairchildsemi.com/ds/TI/TIP126.pdf [Dec. 9, 2011] [9] Webmaster. (2006 March). “ Optical frequency combs.” NIST. [Online]. Available: http://www.nist.gov/public_affairs/releases/frequency_combs.cfm [Dec. 9, 2011] [10]Thorlabs, Inc (2011, Dec 8). Thorlabs.com - Tutorials [Online]. Available: http://www.thorlabs.de/tutorials.cfm?tabID=26065 [11]Images Scientific Instruments (2011, Dec 8). The P-N Junction [Online]. Available: http://www.imagesco.com/articles/photovoltaic/photovoltaic-pg3.html [12]B. Jalali. (2011, Dec 8). Physics and Technology Forefronts: Silicon Lasers [Online]. Available: http://www.aps.org/publications/apsnews/200603/forefronts.cfm [13]S. M. Goldwasser. (2011, Dec 8). Sam's Laser FAQ - Diode Lasers [Online]. Available: http://www.repairfaq.org/sam/laserdio.htm
44 [14] Mode-locking. Wikipedia, the Free Encyclopedia. [Online]. Available:
45 Bibliography
M. Takamoto. (2005). An optical lattice clock [Online]. Available: http://www.nature.com/nature/journal/v435/n7040/abs/nature03541.html
R. Holzwarth. (2001). Optical Clockworks and the Measurement of Laser Frequencies with a Mode-locked Frequency Comb[Online]. Available: http://ieeexplore.ieee.org/xpls/abs_all.jsp? arnumber=970894&tag=1
Kaertner, Franz X. (2006, Oct. 2). Mode-locked Laser Theory [Online]. Available: http://frog.gatech.edu/Mode-locking%20theory%20Kaertner.pdf
Mode Locking: Optipedia, Free Optics Information from SPIE. SPIE - the International Society for Optics and Photonics. [Online].Available:
Dreyer et al., “Laser Phase and Frequency Stabilization Using an Optical Resonator,” Applied Physics B Photophysics and Laser Chemistry 31.2 (1983): 97-105
Bachman et al., “High Power Diode Lasers: Technology and Applications,” Hamburg., Springer, 2007
46 Appendix A. Abbreviations
DFB: Distributed Feedback Lasers
DH- double heterostructure
ECDL: External Cavity Diode Laser
LASER: Light Amplification by Stimulated Emission of Radiation
LD: Laser Diode
QWLD: Quantum Well Laser Diode
TEC: Thermoelectric Controller
VCSEL: Vertical Cavity Surface Emitting Laser
JILA: Joint Institute for Laboratory Astrophysics
CU: Colorado University
SA: Saturable Absorber A-47 PZT: Piezoelectric Transducer
OC: Optical Coupler
DL: Diode Laser
A-48 Appendix B. Budget
The only expenses we have had so far were used to purchase the components needed to build each of the circuit. We ordered the components from digikey and the amounted to the values as follows:
1) Current Controller Parts: $138.43
2) Thermoelectric Cooling Parts: $76.15
3) Power Supply Parts: $133.77
4) Loop Electronics Parts: $109.31
The total cost for the components we have purchased so far comes to a total of $457.67.
$350 of the cost is being covered by the senior design budget and Prof. Randy Bartels has agreed to help us with the rest of the costs.
Next Semester:
We are planning on purchasing PCB boards next semester to build our circuits on. The estimated cost of these is $50 a piece which comes to an estimation of about $200 more. One other major expense we still have other than the PCB is two EMCO model G03 which we are currently looking for more affordable alternative replacement parts. The reason we are looking for replacements for these components is because we need two of them and they cost approximately $60 each.
B-49