Electrically Tunable Mode-Locked Lasers Based on Time-To-Wavelength Mapping for Optical Communications

Laser Original

Electrically Tunable Mode-Locked Lasers Based on Time-to-Wavelength Mapping for Optical Communications

Kohichi R. TAMURA NTT Corporation, NTT Network Innovation Laboratories, 1-1 Hikari-no-oka, Yokosuka, Kanagawa 239-0847

(Received August 20, 2001)

The tunable mode-locked laser based on time-to-wavelength mapping is a relatively unexplored alternative to tunable sources that can be applied to transmission and wavelength switching in optical networks. These lasers have the advantages of a simple tuning mechanism, good wavelength reproducibility, and simple cavity design. The state of research is discussed.

Key Words: Tunable laser, Mode-locked laser, Fiber Bragg grating, Arrayed waveguide grating

1. Introduction ment to make a cavity with a length that depends on wavelength. The oscillation wavelength is selected by changing a single elec- Compact and tunable lasers are considered to be key compo- trical control signal - the RF frequency. The dependence of the nents for realizing advanced functionality in optical communi- oscillation wavelength on RF frequency is determined by the cation networks.1) A variety of designs exist, and, currently, sev- delay element and should be stable over time if a stable wave- eral of them are in the early stages of commercialization. The length-selective element is used. Inherent drawbacks to ETMLs first application of tunable lasers will be as temporary sources are the changing repetition rate with wavelength and the broad for DWDM terminals, which eliminates the need for stocking optical spectrum that results from mode-locking. Even so, as spare parts at each wavelength. While the tuning speed is not we describe below, these lasers may find applications in optical critical in this case, more advanced applications call for speeds switching within nodes or for short distance transmission in ranging from millisecond-to-nanosecond time scales. For ex- LANs or MANs. They can also serve as a device for discrimi- ample, for fast provisioning, protection, and restoration of opti- nating subcarrier labels in optical networks.6) cal paths, switching speeds must be on the order of a millisec- Although the oscillation characteristics of ETMLs have been ond.2) In more advanced network designs, where data is switched examined by several research groups, only recently, has their in units of bursts or packets, switching speeds must be in the use as a tunable source for communications been seriously ex- nanosecond range.3) Using a tunable laser in conjunction with plored. In this paper, we review these devices and discuss some wavelength routing is one way to implement fast switching. Fi- of the hurdles that remain for them to become useful in real sys- nally, there are novel uses of chirped or wavelength swept sources tems. where time-to-wavelength mapping is exploited to form multiple wavelength transmitters using a single data modulator.4) 2. Operating Principle The common approaches to tunable lasers are variations of dis- tributed Bragg reflector (DBR) lasers and external cavity lasers The basic cavity of an ETML is shown in Fig. 1. It contains (ECL).1,5) In DBR lasers, tuning is realized by current injection amplitude modulation (AM) for mode-locking, gain, and a wave- and temperature changes. In ECL lasers, it is usually realized length-dependent delay element (wavelength selective mirrors), using a tunable intracavity filter, such as a diffraction grating or in which the propagation delay changes with wavelength. When Fabry-Perot. Of these tuning methods, only current variation the laser is mode-locked (ML), only the wavelength that is syn- lends itself easily to tuning speeds in the nanosecond range. chronized to the modulation frequency oscillates. Mechanical or thermal methods are limited to millisecond rates. Unsynchronized wavelengths experience excess loss and are However, tuning via current injection in DBR lasers is compli- extinguished in the steady state. cated by the fact that most designs use multiple DBRs and re- The resonance modes fm of the laser can be described by Eqn. quire as many as 3 tuning currents which must be precisely ad- (1), where the mode-locking order m > 0 is an integer, c is the 5) justed to select a given wavelength. The complicated wave- speed of light, and Lo(λ) is the round trip optical path length length selection process along with changes in the current-wave- (effective cavity length) which depends on wavelength. The length mapping characteristic due to aging make reliable and strength of the fast tuning a challenge.

One kind of ECL, which has received less attention, is the fmcLmo()λλ= () ()1 electrically tunable mode-locked laser (ETML) based on time- to-wavelength mapping.6-15) These lasers use a dispersive ele- dependence of Lo on λ is a critical issue in the design of these

28 The Review of Laser Engineering January 2002 Topical Paper

locked. The effects of optical supermode hopping must still be examined; however, in preliminary observations in the labora- tory, they were observed to have a less severe effect on data transmission than mode-hopping in a cw laser.

3. Demonstrations of ETMLs

Although the goal was not wavelength tuning, the first ETML suitable for optical communications was reported in Ref. 7). A Fig. 1 Basic operating principle of a ETML. semiconductor optical amplifier (SOA) was used for gain and AM, and an external cavity was formed using a chirped FBG. Tuning via changes in the driving frequency was observed, lasers, as this parameter along with the time gating by the AM though the goal of the research was to emit chirp-free solitons. determines the wavelength selectivity in a given RF frequency An FBG-based laser of a similar construction that was built spe- range. There are limits to the maximum allowable length change cifically for wavelength tuning was reported in Ref. 8) . Also, in with λ that can reasonably be allowed. One factor that sets a addition to chirped FBGs8,11), discrete FBGs with different limit on the length change is the need to ensure that no more wavelengths12)and highly dispersive fibers. 9,10) were used by than one wavelength is resonant at each RF frequency. Con- various researchers in the demonstration of devices that tuned sider a device operating at the mode-locking order m. The sim- by the same mechanism. In some cases, fiber amplifiers were plest to way to ensure that there is a unique resonance associ- used in place of SOAs10,11); however, their slow gain relaxation ated with each wavelength is to satisfy the relationship of Eqn. times and long lengths make them unsuitable for use in compact (min) (max) (2), where Lo and Lo are the shortest and longest effec- and rapidly tunable sources. tive cavity lengths, respectively. This ensures Figure 2 shows the configuration of a ring cavity ETML, which we have used to study the basic tuning and mode-locking char- mL()max m L () min oo<+()1 ()2 acteristics when using an FBG. The cavity consists of a com- that there is no overlap between adjacent mode-locking orders, mercial electro-absorption modulator (EA) connected to a DC hence the resonance frequencies all have unique values. bias source and RF signal generator (SG), semiconductor opti- Within the constraint of Eqn. (2), the choice of m is a second cal amplifier (SOA), and optical circulator. The FBG had a lin- important design parameter and is related to the λ−dependent ear chirp, length of 3 cm, and a reflection bandwidth of 15 nm. length change in the cavity. For a fixed AM gating characteris- Its reflectivity was approximately 60 %, and the light transmit- (max) (min) tic, it is advantageous to increase ΔLo = Lo - Lo as large ted through the FBG served as the laser output. Figure 3 shows as possible, as this improves the wavelength selectivity. How- the tuned spectra and the dependence of the channel on RF fre- ever, if the fractional change in the cavity length is too large, the quency. Although the laser is continuously tunable, the number spread in the RF frequency range required for tuning becomes of distinguishable wavelengths is set by the spectral width of undesirably broad. Let us define the fractional change F as 0.25 nm. Figure 3 shows 8 distinct wavelengths separated by (avg) (avg) (max) (min) ΔLo /Lo , where Lo = (Lo + Lo )/2. If we operate 200 GHz. The temporal duty cycle at each wavelength was ap- with fundamental mode-locking (m = 1) and set ΔLo to its limit proximately 25 %, and the output power was approximately 0 as given by Eqn. (2), we find that F = 2/3. It is easy to show that dBm. this corresponds to a need for around a 67 % change in the repetition rate around a central operating frequency. If sinusoidal 4. Data Transmission Using ETMLs modulation is used, the large change in the AM gating characteristic with frequency may create undesirable performance dif- Due to the change in repetition rate with wavelength, special ferences as the laser is tuned. There are also practical difficul- schemes must be adopted if a ETML is to be used for data trans- ties with fundamental mode-locking if the target repetition rate mission. One solution is to fix the repetition rate to match the range is high (i.e. several gigahertz and above) because very date rate. This is the conventional way that a mode-locked laser short cavity lengths are needed. is used for data communications at fixed rates, where each pulse One may both relax the need for a short cavity and decrease corresponds to a data bit. Tuning at a fixed frequency can be the RF frequency spread by operating at higher m. At very large realized by physically or thermally varying the cavity length.10,14); m and maximized ΔLo, the RF frequency spread is approximately however, this does not lend itself to fast tuning. Another ap- fc / m around a center frequency fc. ΔLo, in this case, is approxi- proach is to use two time gates within a single cavity, where mately equal to the pulse spacing in free-space that is associated variation in the phase between the gates selects the wavelength.15) with a pulse train at repetition rate fc. A narrow RF frequency range is desirable for relaxing the requirements on the driving electronics. However, there is a trade off in that a large m re- quires a longer cavity length, which increases the device form factor, increases the susceptibility to environmental instabilities, and decreases the tuning speed. Another issue that is worthy of consideration is that of mode-hopping. Mode-hopping in the case of a mode-locked laser amounts to a hopping between different optical supermodes within the same wavelength band. Fundamental mode-locking eliminates the possibility for supermode hopping because all adjacent resonance modes are Fig. 2 ETML ring laser using linearly chirped FBG.

Vol.30, No.1 Electrically Tunable Mode-Locked Lasers Based on Time-to-Wavelength Mapping for Optical Communications 29 Fig. 5 Eye pattern and corresponding BER measurement as repetition rate was varied for laser of Fig. 2. Wave- length fixed at 1552 nm. Fig. 3 RF frequency dependence of tuning (top) and output spectra (bottom) for laser of Fig. 2. comparison. The BER measurements corresponding to each case This approach increases the complexity of the device, and its show that at 5 GHz, the power penalty is less than 0.2 dB, and at practicality has yet to be demonstrated. 6 GHz and higher, there is no observable difference in sensitiv- An unconventional but simpler way that these lasers can be ity compared to the cw source. These results may vary depend- used for data communications is to separate the transmission ing on the duty cycle of the laser output or on the electrical fil- rate R from the repetition rate.6) By setting R to be sufficiently tering / limiting characteristics of the receiver. However, setting lower than fm , the data is carried at a fixed rate as the envelope fm > 2R should serve as an approximate and minimum design of the pulse train. There are obvious disadvantages to such a criterion for error-free operation. scheme, such as a reduced dispersion tolerance and potential Another important consideration in ETMLs is the width of power penalties due to the pulsed nature of the output signal. the output spectrum. Mode-locking naturally tends to shorten However, the simplicity of the construction of such lasers offers the pulse and spread the width of the spectrum, however, a nar- the potential to realize an extremely low cost, rapidly tunable row spectrum is desirable for improving the dispersion toler- source, which may find use in shorter reach applications, such ance and decreasing the wavelength spacing. The usual expres- as in the metro area or local area networks. sion given by Eqn. (3)16) that describes power penalty as a func- Figure 4 shows the bit error rate (BER) measurements at tion of total dispersion DT, source line 2.48832 Gbps when using the laser of Fig. 2. The direct output 2 Power penalty= −−514log RsD 3 of the laser was modulated with a pseudorandom bit sequence 10[]()T () (PRBS) of length 231-1 and detected with a commercial non- width s, and bit rate R can be applied to ETMLs. Figure 6 shows return-to-zero (NRZ) format avalanche photodiode (APD) re- the BER measurements for when the laser of Fig. 2 was used for ceiver to make the BER measurements. Despite the proximity data transmission at 2.48832 Gbps over 25 km of single mode of the R and fm , error-free operation was obtained at each wave- fiber (SMF) which has a dispersion of 16.5 ps/nm/km. The ex- length, which attests to the stability of the laser. perimentally observed power penalty is approximately 2 dB, An important system design consideration is the relationship which agrees well with the value predicted by Eqn. (3). In order that can be tolerated between R and fm. Figure 5 shows the re- to improve the dispersion tolerance, it is desirable to find ways sult of modulating the output of a ETML at 2.48832 Gbps and to narrow the optical spectrum. One way that the FBG can be observing the eye pattern after envelope detection as a function designed to increase the strength of the spectral filtering is to of laser repetition rate. The envelope detection was realized by modify the apodization function to introduce stronger bandwidth using a 1.866 GHz Bessel low pass filter. The eye closure is limiting in the cavity. Figure 7 shows one example of this ap- severe at repetition rates of 3 - 4 GHz; however, at 5 GHz and proach, where a periodic apodization is applied to the same lin- higher, the eye opens. The case of a cw source is shown for early chirped phase mask that was used to fabricate the chirped

Fig. 6 BER measurements at 2.48832 Gbps for laser of Fig. 2 after propagating over 25 km of SMF. Squares: chirped FBG. Circles: chirped FBG with local Fig. 4 BER measurements at 2.48832 Gbps for laser of Fig. 2. apodization.

30 The Review of Laser Engineering January 2002 Topical Paper

FBG in Fig. 2. The reflection spectrum of the FBG now contains local peaks at which the laser operates. The mode-locked spectrum is narrowed (0.18 nm), and the improved dispersion tolerance is shown in Fig. 6. This is obtained, however, at the expense of continuous tuning, as the laser now tunes in discrete hops and with a limited stable locking range.

5. ETMLs Based On AWGs.13)

Because the reflection bandwidth of an FBG is inversely pro- portional to its length, it is difficult to fabricate a short FBG that has many narrow bandwidth reflection windows. Also, the group delay ripples of FBGs make fine delay control difficult. An alternative delay element that can be used is an arrayed waveguide grating (AWG), which separates each wavelength channel into Fig. 9 Channel vs. RF frequency (top) and spectra (bottom) separate waveguides. The length of each waveguide is adjusted from 32 wavelength AWG ETML of Fig. 8. to set the delay, while the spatial dispersion of the AWG inde- pendently sets the channel spacing. Figure 8 shows the experimental setup that we used to perform a proof-of-principle demonstration using a 32 channel AWG which had a channel spacing of 100 GHz. The AWG was fabricated using silica-on-sili- con planar lightwave circuit technology. The equivalent free- space delay difference between adjacent wavelengths was set to 0.5 mm, which is suitable for a laser operating in the 10 GHz regime. The gain and AM were obtained from an integrated InGaAsP EA / SOA which was high reflection (HR) coated on one side and antireflection (AR) coated on the other (R < 0.1 %). Fig. 10 Expanded view of the spectrum of a single chan- The polarizatoin controller (PC) set the cavity polarization and nel. Dashed trace is AWG filtering characteristic. the percentage of output coupling at the polarization beam split- ter (PBS). The PBS fixed the polarization entering the AWG, which had a polarization dependent response. uted to the coupling losses in the cavity, a low output coupling The tuning results in the 10 GHz regime are shown in Fig. 9. percentage at the PBS, and a low injection current to the SOA. Figure 10 shows an expanded view of the spectrum of one chan- It is important for these lasers to be tunable by only changing nel to show the spectral fill-factor within the AWG passband. the RF frequency. Figure 11 shows the SOA injection current, The spurious oscillation of other channels is typically below -25 RF power to the modulator, and temperature to the InGaAsP dB. The pulse width varied between 30 - 40 ps across the tuning chip as the laser was tuned over its entire tuning range. The bias range due to the wavelength dependence of the EA. The output to the EA was fixed at -2 V. Except for a change in temperature power was approximately -17 dBm. This low power was attrib- at longer wavelengths, Fig. 11 shows that tuning via changes in the RF frequency alone was achieved over a large part of the tuning range. The temperature change that was needed at longer wavelengths served to shift the gain spectrum of the SOA to favor oscillation in this region. Controlling the Fabry-Perot resonance-induced gain ripples in the InGaAsP chip was critical to achieve tuning via changes in the RF frequency alone. If a channel fell on a gain minimum, oscillation of that channel was ei- ther unstable or would not initiate. Eliminating the ripples proved Fig. 7 Illustration to show effect of local and normal to be difficult due to limits on the AR coating; hence, we chose apodization on chirped FBG reflectivity (left). Ex- to use a chip that had a free spectral range (FSR) of 50 GHz, perimentally observed output spectra for each case which matched peaks in the gain to the AWG passbands. In Fig. (right). 11, we show for comparison the temperature changes that were required at each wavelength when using a chip with a 40 GHz FSR. In this case, every other channel falls on a gain peak, hence, variation of only the RF frequency causes the laser to skip channels. However, since the wavelengths of the ripple peaks can be shifted by changing the temperature, temperature adjustments can be used to force the laser to operate at all wavelengths (' × ' in Fig. 11).

6. Conclusions

Fig. 8 Experimental configuration used to test AWG-based The ETML is a relatively unexplored alternative to tunable ETML.

Vol.30, No.1 Electrically Tunable Mode-Locked Lasers Based on Time-to-Wavelength Mapping for Optical Communications 31 Acknowledgments

The author gratefully acknowledges guidance, helpful discus- sions, and collaboration with Tetsuro Komukai, Kenji Sato, Yasuyuki Inoue, Akio Sugita, Masataka Nakazawa, Kenichi Sato, Kunihiko Mori, and Toshio Morioka.

References

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32 The Review of Laser Engineering January 2002