Self-Q-switching and mode locking in a 1.53-btm fiber ringlaser with saturable absorption in erbium-doped fiber at 4.2 K
Masataka Nakazawa, Kazunori Suzuki, Hirokazu Kubota, and Yasuo Kimura
Optical Transmission Line Laboratory, NTT Telecommunications Field Systems Research and Development Center,Tokai, Ibaraki-ken 319-11, Japan
Received November 23, 1992
Self-Q-switching and mode locking have been observed for the first time to our knowledge in a 1.53-,tm erbium-
doped fiber ring laser that has an erbium-doped fiber cooled to 4.2 K as a saturable absorber. The repetition
period of the self-Q-switching was 77 As (13 kHz), which was determined by a gain recovery time of the laser.
Cw mode locking was also observed when the pump power was increased to 150 mW. The Q-switching pulse
width was approximately 3.3 As, and the shortest mode-locked width was approximately 20-30 ns.
Erbium-doped fiber amplifiers (EDFA's) have gainedmuch attention as key components for optical com-munications because of their polarization-insensitivehigh gain, low insertion loss, and low noise character-istics.' They are also useful for experiments oncoherent pulse propagation such as self-inducedtransparency2 (SIT) and coherent -r pulse propaga-tion.3 SIT offers the possibility of pulse shapingand standardization that is different from non-linear Schrbdinger soliton formation.' In our SITexperiment in an erbium-doped fiber (EDF), wedemonstrated 2ir pulse propagation and the breakupof a SIT pulse at 4.2 K.5 In addition, by opticallypumping the EDF with 1.48-Am InGaAsP laserdiodes, coherent ff pulse propagation was observed,which was accompanied by many pulse ringings.6
During these experiments, the idea arose of form-ing a SIT soliton laser with an EDFA at room tem-perature as a gain medium and an EDF at 4.2 K as apulse shaper by using the SIT effect. In this Letterwe report the self-Q-switching and mode locking of a1.53-pum erbium fiber laser at 4.2 K.
The experimental setup is shown in Fig. 1.The unidirectional ring laser with a polarization-insensitive optical isolator consists of two importantparts. One is an EDFA as a gain medium, which waspumped by a Ti:sapphire laser at 0.98 ,um. The EDFhad a doping concentration of 200 pm and was 100 mlong. The other part is a cooled EDF at 4.2 K. Thecooled EDF had a doping concentration of 8900 partsin 106 and was 3 m long. The cutoff wavelength, zerodispersion, and relative refractive-index differencewere 1.26 /um, 1.53 Am, and 1.3%, respectively. Toobtain laser oscillation at the resonant wavelength, a1.53-1um optical filter with a bandwidth of 1 nm wasinstalled. The total cavity length was approximately135 m, corresponding to a pulse repetition rate of1.53 MHz. The output pulse was extracted by a 8:2fiber coupler with a 20% output coupling and detectedwith an InGaAs p-i-n photodiode.
The linear absorption spectrum of a 21-cm-longEDF is shown in Fig. 2(a), in which the dashed andsolid curves are absorption spectra at room tem-
perature and 4.2 K, respectively. The absorption at1.530 /um was 25 dB. Since we used a 3-m-longEDF, the total linear absorption at 4.2 K was as highas 357 dB. Figure 2(b) shows absorption change ver-sus average input power to the fiber at room tem-perature and 4.2 K. At room temperature, the ab-sorption at 1.534 /-tm remains unchanged for inputsbetween -50 and -20 dBm, but it changes drasti-cally when the EDF is cooled to 4.2 K, at which tem-perature absorption bleaching (saturated absorption)occurs. When the coupled intensity I is much lowerthan 'sat (= h V1/-r), the absorption coefficient is com-pletely unbleached and linear at 4.2 K. However,when I is higher than Isat, the absorption is given asa hcoo(N1 - N2)/2T1 I. Here coo is the resonancefrequency, T1 is the population relaxation time, andN1 - N2 is the population difference. Thus theabsorption decreases in inverse proportion to I, asshown in Fig. 2(b). The absorption bleaching mayrealize self-Q-switching and mode locking as was
Erbium-DopedFiber
Optical Optical
ALTi:Sapphire A DAr Laser Laser
8:2 Coupler
High-Speed
Liquid
Analyzer e_ Erbium-Doped
Optical ~~~~FiberAnalyzer Cryostat System
Fig. 1. Experimental setup for self-Q-switching andmode locking of a 1.53-/,tm fiber ring laser with saturableabsorption in EDF. WDM, wavelength-division multi-plexing.
Fig. 2. Absorption characteristics of an EDF. (a) Lin-ear absorption spectra of the erbium-doped fiber at roomtemperature (R.T.) and 4.2 K. (b) Changes in absorptionversus average cw input power at R.T. and 4.2 K.
reported in the 1960's with saturable dyes or glassfilters.7' 9
If a sharp noise pulse, whose pulse width is shorterthan the dipole relaxation time (transverse relaxationtime) T2, builds up in the cavity and the gain ispositive in the loop, it eventually becomes a SIT pulsewith a pulse area of 2v. When T2 is shorter thanthe pulse repetition rate that is determined by thecavity length, a coherent pulse oscillation would beobserved. Here we estimate the amount of power re-quired to achieve a SIT oscillation. Assuming a typ-ical linewidth AAH of 3 nm (AVH = AHc/A2) and o- =5 x 10-25 M2 ,10 one obtains IP211 = 1.4 x 10-32 Cm =4.7 X 10-3 D, where A = 1.55 ,um and e0 = 8.85 X10-12 F/m. The peak intensity of a 27r pulse Ipeakis given by Ipeak = 1/2cneo(1.76)2(h/IP 2l rF)2 W/m2,where TF (= 1.7 6T,) is the FWHM of a sech SIT pulse.When the EDF is cooled to 4.2 K to prolong T2 , T2becomes a few nanoseconds." For TF = 500 PS, PSITfor EDF's is calculated to be as low as 107 W, asdescribed in Ref. 5. Thus the average power forsuch a SIT pulse is estimated to be 80 mW for arepetition rate of 1.5 MHz. Since such a power levelis achievable in a fiber laser, there is the possibility ofrealizing a SIT laser. Whether the laser operates asa SIT soliton laser or is a self-Q-switched and mode-locked laser is determined by whether the coherentor the incoherent absorption is smaller.
The temperature dependence of the self-Q -switchedpulse oscillation that we obtained is shown in Fig. 3.Figure 3(a), top, middle, and bottom, correspond to4.2, 10, and 18 K, respectively. The correspondingoscillation spectra are shown in Fig. 3(b), top, middle,
and bottom, respectively. The strongest pulse oscil-lation took place at 4.2 K, and the oscillation becameprogressively weaker with increases in the fibertemperature. Above 18 K, the oscillation stoppedcompletely because the absorption became extremelylarge, as expected from Fig. 2(b). The pulse rep-etition period was approximately 77 Aus (13 kHz),which was determined by the gain recovery time(100-200 us) of the EDFA at room temperature.' 2
The repetition period changed to 55 PAs when thetemperature was changed to 10 K. There was nopulse oscillation when the cooled EDF was removed,and only unstable cw oscillation was observed.
Figure 4 shows details of the oscillation character-istics when the gain of the laser cavity is changedby increasing the 0.98-pum pump power to the room-temperature EDFA. The pump powers were 70 mW[(a) and (b)], 80 mW [(c)], 90 mW [(d) and (e)], and100 mW [(f)]. When the pump power was low, apulse width was as wide as 3.4 Us. Since this pulsewidth is much broader than T2, it can be said thatthe pulse was produced by the bleaching (saturableabsorption) in the cooled EDF at 4.2 K. In addi-tion, as is common in a Q-switched operation, aslight asymmetry with a steeper rise and a relativelyslower fall was observed, with a peak power of ap-proximately 20 mW. The self-Q-switching was notstable and the waveform changed into that shown inFig. 4(b) for the same pump power. A deeper inten-sity modulation can be observed for a pump power
dBm
-35
-551.5 1.55 1.6
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dBm-15 r
-35
155 L.51.5
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-35
-55
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(a) (b) JimFig. 3. Temperature dependence of the laser oscillation.(a) Top, middle, and bottom correspond to 4.2, 10, and18 K, respectively; (b) corresponding oscillation spectra.The time scale in (a) is 50 Uis/division.
(C) (M)Fig. 4. Changes in oscillation waveform with increasesin the 0.98-1ttm pump power. The pump power was (a),(b) 70 mW, (c) 80 mW, (d), (e), 90 mW, and (f) 100 mW.The time scale in (a)-(e) was 1 tus/division, and that in(f) was 500 ns/division.
(a) (c)
(b) (d)Fig. 5. Oscillation waveform changes in cw mode lock-ing. The pump power for (a) was 150 mW and that for(b) and (c) was 170 mW. (d) is an electrical beat signal.The time scale in (a)-(c) was 500 ns/division, and thefrequency scale in (d) was 0.5 MHz/division.
of 80 mW, as shown in 4(c), in which the modula-tion period was approximately 660 ns, correspondingto the cavity transit time. Further increase in thepump power led to narrower pulses, which are dueto the self-mode-locking of the pulses. Self-lockingoccurs when two or more longitudinal modes begin
oscillating, and their mutual interference causes aperiodic amplitude fluctuation at a frequency equalto the spacing between the modes. These intensitymodulations can be enhanced through the saturatedabsorption in the EDF. As seen in Figs. 4(b)-4(f), ahigher pump power causes many modes, resulting insharper pulses. Figures 4(e) and 4(f) correspond toharmonic mode lockings. A similar experiment wasreported for a Q -switched ruby laser with a saturablefilter. 8' 9
By further increasing the pump power, cw modelocking was observed, as shown in Fig. 5. Thepump power for Fig. 5(a) was 150 mW and that forFigs. 5(b) and 5(c) was 170 mW. Figure 5(d) showsself-beat signals between the longitudinal oscillationmodes in 5(a), measured with an electrical spectrumanalyzer. Cw mode locking can be clearly seen inFig. 5(a), in which the repetition rate was 1.53 MHz.When the pump power was changed, a differenttype of harmonic mode locking was observed, asshown in Figs. 5(b) and 5(c). A frequency separationof 1.53 MHz in the beat spectrum of Fig. 5(d)corresponds to a cavity round-trip transit time of660 ns. It is also noted that self-Q-switching andmode locking did not occur when the EDF at 4.2 Kwas removed from the cavity. The minimum pulsewidth was approximately 20-30 ns, which was stillbroader than T2. Thus we conclude that it is possibleto obtain passive Q switching and mode lockingby using a cooled EDF. However, the pulse widthwas broader than T2, which means that saturableabsorption rather than a coherent pulse formationwas dominant in the present experiment. A shortercooled EDF may yield SIT mode-locked operation.
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