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AN-LD18 Optimizing Laser Diode Control

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AN-LD18 Optimizing Laser Diode Control Application Note AN-LD18 Rev. A Optimizing Laser Diode Control November, 2020 Page 1 INTRODUCTION This application note will provide a practical step-by-step guide to optimizing laser diode control with rule of thumb Laser diodes are used in a variety of applications, each approximations that work with most laser diodes. This will utilizing the unique properties of semiconductor lasers. show the recommended operating techniques of laser Creating effective systems can be complicated, but well diodes with laser diode drivers for the optimum results. worth the effort of optimization. Laser diodes are used in wide-ranging fields such as space research, spectroscopy, As always, some laser diodes may be different and telecom laser manufacturing, medical lasers (surgery and require special considerations. We have included a basic diagnostics), gas measurements, LiDAR (Light Detection troubleshooting guide that covers the majority of problems and Ranging), optofluidics, microfluidics, metrology, life our customers have encountered over time. sciences, and many more. Laser diodes are compact and reliable. Extremely low noise SYSTEM COMPONENTS and stable output wavelength can be achieved with laser In a typical laser diode system, a driver (current source) diodes using the proper techniques and design. Laser is used to control the current from the power supply to the system integrators must have a good understanding of the laser. Figure 1 shows the basic layout of a laser driver application and how the laser diode and laser driver fit into system. The driver uses a feedback system from the laser the system. Optimized diode control will reduce wavelength or photodiode to correctly and accurately operate the laser. instability, noise produced and added to the system, and Application Note AN-LD13: Laser Diode Driver Basics goes keep the user safe to operate the equipment. into further detail about laser diode drivers. In the following sections, different aspects of the laser driver will be reviewed in detail for high performance operation. GND User VDD Inputs VS Limit Bronout, To poer slo start, ED control Error delay, Photo Limit Transient electronics enabledisable, Diode Circuit Protection other safety functions Laser User PD LD Diode Chooses Current Current Feedback ode Adjustable Current ource User Onboard G(Poer) G(Current) Inputs etpoint Control Control etpoint Circuit ystem ystem Ammeter A ION Error + + + - Compare etpoint to Actual Create umming Error etpoint Amplifier Ammeter odulation A PON Input Compare + etpoint - to Actual Figure 1. Laser Diode Driver Block Diagram © 2020 • Sales & Technical Support: (406) 587-4910 • email: [email protected] • web: www.teamwavelength.com Application Note AN-LD18 Rev. A Page 2 TEMPERATURE CONTROL diode’s datasheet for recommended operating temperature range. Although laser diode drivers are important to operating a laser diode, temperature control is also vital to correctly and safely using a laser diode. The output of a laser diode, like other electronics, is affected by its ambient temperature. In 0 the case of laser diodes, a change in temperature can shift 40 the wavelength of the output of the laser. This can occur 50 when there is an increase in injection current or a change of ambient temperature (Figure 2). Power ▪ 100 ▪ 150 ▪ 200 Current ▪ 00 Figure 3. Output Power vs. Input Current with Multiple ower Temperatures Laser diode drivers can also have their output current affected by temperature. Most datasheets will list a temperature coefficient for the driver in ppm/ºC (parts per million per degree Celsius). As temperature changes, the 62 6 64 65 66 67 68 output current of the driver will slightly shift. Typical values Wavelength (nm) range from 25 to 300 ppm/ºC, but these are still relatively Figure 2. Intensity vs. Wavelength with Multiple small changes compared to the total output of the system. Temperatures For example, a laser driver with output current of 1.5 A and temperature coefficient of 300 ppm/ºC will produce a shift of The wavelength shift may not be large, but it could affect 0.045 A for a 100ºC temperature change. delicate laser setups. The change in temperature will affect the bandgap energy, the refractive index of the laser Technical Note TN-TC01: Optimizing Thermoelectric diode, and therefore the peak wavelength. When there is Temperature Control Systems shows practical temperature an increase in temperature, output power will decrease and control techniques using temperature controllers and TECs. the threshold current of the laser diode will increase. Like most electronics, laser diodes are not 100% efficient with the FEEDBACK MODE SELECTION supplied power. There will be heat dissipation within the laser, changing the characteristics of the output. Controlling the Most laser diode drivers will allow control based on laser temperature of the laser will control the output. Commercial current or photodiode current feedback. These are labeled temperature controllers have the ability to control the as Constant Current (CC) and Constant Power (CP), temperature using Thermoelectric Coolers (TECs) to as low respectively. This will allow the driver to maintain the as 0.0009ºC precision. The coolers will help to control the specified output as constant as possible. Figure 1 shows ambient temperature around the laser as well as the internal the basic layout of both feedback systems (yellow and gray temperature of the laser due to the input current. feedback modes). The output power of the laser can also be affected by the CC mode will use the current through the laser diode as temperature of the laser. Figure 3 shows a plot of output feedback into the system. If the current is not constant, power of a laser diode versus input current. The threshold an error will be produced in the design circuity to drive the current (current at which stimulated emission and coherent current back to the setpoint. light is produced) increases as temperature increases, and the output current decreases with a fixed input current. As CP mode will use the current through the photodiode as the temperature cools, the threshold current decreases. feedback into the system. The photodiode monitors a This is why many experiments maintain lasers at cooler fixed portion of the light generated by the laser diode. The temperatures, increasing the efficiency. Refer to the laser photodiode manufacturer can provide a rough transfer © 2020 • Sales & Technical Support: (406) 587-4910 • email: [email protected] • web: www.teamwavelength.com Application Note AN-LD18 Rev. A Page 3 function relating the photodiode current to the laser diode 50 nV/√Hz. The noise floor at Wavelength is ~15 nV/√Hz at power output (mA/mW or µA/mW). The photodiode current 1 kHz. A pre-amplifier should be used to bring the device is linearly related to the optical power of the laser diode. The under test’s (DUT) noise to an amplitude well above the driver will adjust the current to the laser diode to keep the noise floor of the system. A battery powered pre-amplifier optical power level constant. should be chosen to keep noise at a minimum. The system will back out the pre-amplifier gain and measure the noise of the device. The noise density of the system at Wavelength CHOOSING CONSTANT CURRENT OR CONSTANT is ~4 nV/√Hz. It will be common to see a spike around the POWER MODE 60 Hz power line noise. In some cases, constant wavelength is critical, but having a wavelength meter provide feedback is not practical. The Not all noise can be eliminated or reduced. The physical laser can be driven in constant current mode to keep the properties of materials or the laser diodes themselves will current through the laser constant as well as the wavelength. produce a minimum amount of noise. The lowest level of Because temperature is also critical to stabilize wavelength, noise in resistors is called Johnson voltage noise density. it can be scanned to achieve the desired wavelength, then Johnson noise of a resistor can be calculated using: held constant. √4kBT In other cases, constant power is required. One example is sensing applications to illuminate a sample measuring where k = 1.38 x 10-23 (Boltzmann’s constant), T is absorption, reflection, or scattering. Here the detector senses B power, so a constant input power is necessary to maintain temperature in Kelvin, and R is resistance in ohms. Below calibrated measurements. Another application requiring is an example using a 10 kΩ resistor at 300K. Total noise is added in quadrature. Noise is the noise calculated of the constant power is communications when signal degradation 1 resistor. Noise is the pre-amplifier noise of 4 nV/√Hz. affects signal to noise ratio and system performance. To 2 insure constant power output, feedback from the integrated -2 -4 photodiode is monitored and the laser diode current source √4*(1.810 )*00*10 increases its output until it reaches a preset current limit. This is important as laser diodes lose power as they age = 12.9 nV/√Hz - more current is required to achieve the same power over oise = √(Noise )2 (oise )2 time. Total 1 2 -9 2 -9 2 oiseTotal = √(12.9x10 ) + (4x10 ) NOISE = 13.5 nV/√Hz Noise can become an issue when making high precision or near noise floor measurements. This is unwanted signal Current noise density (nA/√Hz) can be calculated from the or fluctuation in the signal. There are various sources of voltage noise density and the resistance of the load. noise including: cables, laser driver, system connections, power supplies, external electromagnetic interference, and Voltage oise ensit urrent oise ensit the selection of the photodiode. Noise is usually denoted esistance in micro-amps (µA). Spectral noise density (nA/√Hz) can also be measured and shows the performance at specific where the units are: frequencies across a measurement bandwidth.
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