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

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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 mul- tiple 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 rep- etition rate around a central operating frequency. If sinusoidal 4. Data Transmission Using ETMLs modulation is used, the large change in the AM gating charac- teristic 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.
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