The performance of Er3+:Yb3+ single frequency fiber lasers is reported. With lasing powers up to 60mW, they are shown to have excellent specifications in terms of optical linewidth, signal-to-noise ratio, RIN, SMSR and polarisation purity

Cowle, G.J.

Dong, L.

Hsu, K.

Loh, W.H.

Samson, B.N.

English

Southampton (e-Prints Soton)

1 Optoelectronics Research Centre, University of Southampton, UKPerformance Characteristics of Single Frequency Er :Yb  Codoped Fiber Lasers 3+ 3+W.H.Loh, B.N.Samson, L.Dong, G.J.Cowle and K.Hsu* Optoelectronics Research Centre, University of Southampton, Southampton SO17 1BJ, UK *Micron Optics Inc. 2801 Buford Hwy, Suite 140 Atlanta, Georgia 30329, USA AbstractThe performance of Er :Yb  single frequency fiber lasers is reported. With lasing powers up to 60mW, they3+ 3+are shown to have excellent specifications in terms of optical linewidth, signal-to-noise ratio, RIN, SMSR andpolarisation purity.For much of this decade, interest has been by scanning a 100 mW, 244 nm beam across asustained in developing high performance single phase mask. The DFB laser, placed on anfrequency Er -fiber lasers [1-3]. For robust single aluminium plate without temperature control, is3+frequency operation, these lasers need to be only spliced directly to a fiber WDM, which couples thecentimeters long. Early grating-based fiber lasers pump from a 40 mW 980 nm laser diode. Therelied on Er-germanosilicate fibers, but lasing 1550 nm arm of the WDM is spliced to a pigtailedpowers were low - µW typically [1,2], due to low isolator. The laser power measured out of thepump absorption in the short cavity. Increasing the isolator is 4 mW. Er-concentration to increase absorption is howeverdifficult. Germanosilicate fibers are prone to ion At this power, the optical SNR is 65 dB. Aclustering, which degrades the efficiency and scanning F-P interferometer confirmed stabledestabilises [1,5] the laser. Thus the need to single frequency and single polarisation operation.amplify the laser power to useful levels of a mW or The optical linewidth, measured with the delayedmore is a drawback where low noise sources are self-heterodyne technique, is 18 kHz. The RIN is <desired.  -153 dB/Hz for frequencies > 10 MHz. The peakConsiderable effort has been put into investigating frequency of 0.6 MHz, is lower than previous fibersolutions for increasing the power from short lasers using feedback noise reduction circuits [1].cavity fiber lasers [3,6]. While the absorption can Estimating the side mode suppression ratiobe increased by changing the pump wavelength [6], (SMSR) is more involved. The side modes for a 5this is unlikely to be fully practical till the arrival cm DFB laser would be GHz away, too close for anof green laser diodes. On the other hand, Er :Yb optical spectrum analyser with 0.1 nm resolution.3+ 3+fibers are mature. With this scheme, the 980 nm We estimate the SMSR from the rf frequencyabsorption can be increased up to 2 orders of spectrum during the self-heterodyne measurement,magnitude, with corresponding increases in output in the 1-7 GHz frequency range. Any side modespower.  should show up as a beat signal (with the mainHowever, two problems stood in the way. One was observed. From the main mode beat signal level,the low photosensitivity in these fibers. In addition, we conclude that neighbouring side modes are < 50the lasers were observed to operate in 2 polarised dB below the main mode, resolution limited by themodes [7], unattractive for many applications. noise floor. Recently, a Er :Yb  fiber with a photosensitive3+ 3+annular region surrounding the core enabled strong To determine the single polarisation state purity,gratings to be written. Also, the resulting lasers the laser output was connected to a polarisationwere observed to lase in a single polarisation state controller and a polariser. By adjusting the[8]. We report here on the characteristics of these polarisation controller, the output from thelasers, and show their potential as high polariser was reduced by 50 dB. The 2ndperformance low noise narrow linewidth sources. polarisation mode is thus effectively nonexistent.We first present the performance of a 5 cm the Er :Yb  DFB fiber laser. distributed feedback (DFB) fiber laser, fabricatedRIN of -118 dB/Hz, at the relaxation oscillationmode) in that range. No such beat signals wereTable 1 summarises the device characteristics of3+ 3+2 Optoelectronics Research Centre, University of Southampton, UKFig. 1. (Top right) Comparison of BERperformance of DFB fiber and semiconductorlasersOutput Power  4 mW Optical Linewidth  18 kHz Optical Signal-to-Noise Ratio  65 dB Relative Intensity Noise,  < -153 dB/Hz  RIN ( > 10 MHz)Peak RIN (at 0.6 MHz)  -118 dB/Hz Side Mode Suppression Ratio  < -50 dB (SMSR)Single Polarisation State Purity  50 dB Wavelength Stability  0.01 nm/EC Table 1. DFB fiber laser device characteristics Finally, transmission tests were performed in a 10 Fig. 2(b) shows the lasing characteristic of theGb/s NRZ system over 50 km of standard fiber. A FBGL. A maximum power of 58 mW (at thediode DFB laser (with 8 MHz optical linewidth) output of the isolator) is obtained for 500 mWwas also similarly tested for comparison. Fig. 1 pump power. A scanning F-P interferometershows the plots of the Bit Error Rate (BER) confirmed single frequency operation. The RIN isperformances, establishing that no problems or < -168 dB/Hz for frequencies > 10 MHz, with apenalties are incurred in using the fiber laser.  peak RIN value of just -133 dB/Hz at theThese findings show that Er :Yb  DFB fiber optical linewidth was measured to be 500 kHz.3+ 3+lasers are excellent for applications requiring Single polarisation state purity was confirmed tosingle frequency low noise sources. However, some be better than 50 dB. applications, e.g. CATV, require more power.Unfortunately, at high pump powers, these lasers The 60 mW FBGL has a reasonable net efficiencyinvariably fall off in power [8]. These behaviours, of 12% with respect to pump power, but it is clearrepeatable and occurring over time scales of many that the efficiency is much better at lower pumpseconds, are attributed to thermal effects, changing powers. This is due to the 'bottleneck' effect,the refractive index and dephasing the grating. arising from the finite energy transfer rate fromHowever, high power single frequency operation excited Yb  to Er  ions. As the transfer rate is acan be achieved, as described below.  strong function of the ion separation, optimisingFig. 2(a) shows the configuration of a Er :Yb may enable output powers exceeding 100 mW. It3+ 3+fiber Bragg grating laser (FBGL). One end was recently argued that the Yb -Er  energyconsists of a dielectric mirror (R=99.86% at 1.55 transfer can usefully act as a low-frequency filter,µm), while the other end is a centimeter long Bragg buffering Er :Yb  lasers against pump noise [9].grating (R=98% at 1536 nm) written directly in thedoped fiber, and situated a few mm away from thedielectric mirror. The entire laser is < 1.5 cm inlength. relaxation oscillation frequency of 2.8 MHz. The3+    3+the Yb  and Er  concentrations and their ratio3+    3+3+ 3+3+ 3+3 Optoelectronics Research Centre, University of Southampton, UKFig. 2 (a) Configuration of FBGL.Fig. 2(b) Lasing characteristic of FBGLHowever, as this also leads to the 'bottleneck'problem, the degradation in laser efficiency at highpump powers would appear to outweigh theadvantage of any limited immunity against pumpfluctuations. The results clearly show that high performancesingle frequency short cavity fiber lasers are finallya realistic option. They are now capable ofproducing reasonable output powers, and possessexcellent low noise characteristics underfree-running conditions. Only centimeters long,they are compact, easy to fabricate and package. In10 Gb/s NRZ transmission tests over standardfiber, they perform equally well compared to adiode DFB laser. Although the fiber lasers are notnecessarily superior in digital transmission tests,they may hold advantages over DFB diodes. Apartfrom ease of fabrication, fiber lasers have awavelength insensitivity to temperature at least anorder of magnitude better, which could beattractive in dense WDM systems requiring tightwavelength tolerances. Also, as WDMtransmitters, fiber lasers may afford betterreliability, e.g. for a 16-channel WDM transmitter,by first combining the pump powers from all thepump diodes before redistributing them to each ofthe fiber lasers, a failure in any one laser (pump)diode will simply result in only a small reduction inthe transmitter power of each channel. 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G.Hull-Allen and W.W.Morey,

J.E.Caplen, J.D.Minelly, K.Hsu and L.Reekie,

L.Dong and J.E.Caplen,

L.Reekie, J.E.Townsend, G.G.Vienne and D.N.Payne,

O.Svelto and G.DeGeronimo,

Performance characteristics of single frequency Er<sup>3+</sup>:Yb<sup>3+</sup> codoped fiber laser

Performance characteristics of single frequency Er<sup>3+</sup>:Yb<sup>3+</sup> codoped fiber laser

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