666 OPTICS LETTERS / Vol. 13, No. 8 / August 1988

Raman amplification in a P205-doped optical fiber

Kazunori Suzuki and Masataka Nakazawa

NTT Transmission Systems Laboratories, Tokai, Ibaraki-ken, 319-11, Japan

Received January 5, 1988; accepted May 10, 1988

The 1.5-,um-band optical amplification characteristics owing to stimulated Raman scattering in a highly P205-doped single-mode fiber are experimentally investigated. With the combination of a 1.32-,m pump and a 1.59-,umsignal, a net power gain of 15.4 dB at a wavelength of 1.59,um is obtained for a pump power of 15 W. The test fiber isan 800-m-long single-mode fiber in which the core region is doped with 17 mol % P 20 5 . The 3-dB gain bandwidth is23.5 cm-', and the optical power ratio of the amplified signal to the spontaneous emission is about 25 dB for abandwidth of 1 nm at the first Stokes line.

Light amplification using stimulated Raman scatter-ing (SRS) in a single-mode optical fiber is of greatinterest in optical communications because of the highgain and the fast response time. Raman amplificationusing SiO2 , GeO2 , and D2 as gain media has been re-ported.1-3 This process can be applied to the exten-sion of transmission length or to the transmission ofultrashort pulses. A highly P 205 -doped silica single-mode fiber has an intense and sharp Raman scatteringpeak at 1320 cm-', i.e., its frequency shift is threetimes as large as in silica fibers.4 Therefore, 1.5-,tm-band light can be directly generated by SRS with a 1.3-,m pump light source. We have previously reportedthe SRS properties of a P20 5-doped silica single-modefiber and its application to the 1.5-Mm-band opticaltime-domain reflectometer.5'6 However, so far therehas been no detailed report on the characteristics ofRaman amplification in P20 5-doped fibers.

This Letter describes efficient Raman amplificationof a highly P2 05 -doped silica single-mode fiber, inwhich the pump-power dependence of the optical gain,the gain bandwidth, and the noise properties in theRaman amplifier are discussed.

The experimental arrangement used for Raman am-plification measurements of a P205-doped single-mode fiber is shown in Fig. 1. The pump light sourceis a Q-switched Nd:YAG laser operating at 1.32 ,um,and the signal light source is an InGaAsP buried het-erostructure laser diode operating around 1.59 ,im.The pulse widths of the pump and the signal sourcesare 1.0 and 0.1 Asec, respectively. In this configura-tion, the pump and signal light are launched at thesame end of the P205-doped fiber through a dichroicmirror. A time synchronizer is used to achieve syn-chronization between the two pulses. The amplifiedsignal passes through a grating monochromator with1-nm spectral resolution and is detected with anInGaAs p-i-n photodiode. The detected signal is pro-cessed by using a boxcar averager. Raman power gainis measured by comparing the detected power of theamplified signal with that of the input signal. Thewavelength dependence of the Raman gain is mea-sured by varying the emission wavelength of the laserdiode.

The test fiber is a highly P20 5-doped silica single-

mode fiber made by the modified chemical-vapor de-position method. The fiber is not a polarization-maintaining fiber. The P205 concentration in thecore region is 17 mol %, which is determined by mea-suring the refractive-index difference between thecore and the silica jacket layer.5 The fiber length usedin the experiment is 800 m, and the fiber losses at 1.32and 1.59 ,m are 2.8 and 5.7 dB/km, respectively. Theincreased loss at 1.59,um may arise from imperfectionsdue to the high P20 5 concentration. The zero chro-matic dispersion wavelength is shifted to 1.55 gm inorder to suppress undesirable light generation due tofour-photon mixing at zero dispersion.7

The pump-power dependence of the Raman powergain is shown in Fig. 2. The wavelength of the signallight is tuned to 1.597 ,m, where the center wave-length of the first Stokes peak owing to P=O bondvibration lies. The peak input power of the signallight pulse in the fiber is approximately 10 MW (-20dBm). The Raman gain increases exponentially withan increase in the pump power. A 20-dB Raman gainis obtained for a pump power of 15 W, where it is notedthat the net gain becomes 15.4 dB because of therelatively high loss. A 20-dB Raman gain implies again slope of 1.33 dB/W. When the pump power ex-ceeds 12 W, gradual saturation of the gain is observedbecause of pump depletion caused by the generation ofhigher-order Stokes signals. A Raman gain as high as28.5 dB is obtained in the experiment for a pumppower of 26 W. Raman gain exceeding 35 dB at 1.56

DICHROIC P205 -DOPEDFILTER FIBER

1.59pm /< ~~~~InGaAs PD

5L A-ER 1_ IMONOCHR_

{GENERATORg |AVERAGER rFig.1. Experimental setup for Raman amplification in for-ward pump configuration. PD, photodetector.

0146-9592/88/080666-03$2.00/0 © 1988, Optical Society of America

August 1988 / Vol. 13, No. 8 / OPTICS LETTERS 667

20 -z

4

,N1. 597 pm

010 20LPUMP POWER (W)

Fig. 2. Pump-power dependence of Raman gain.

lim has been already reported by using molecular D2dissolved in a silica fiber. The D2 -charged fibersshould be kept at liquid nitrogen temperature to pre-vent the diffusion of D2 gas from the fiber.2

The gain saturation of the P205-doped fiber is weak-er than that of GeO 2 -doped fibers.3 In the GeO2-doped fiber, the Raman gain for the first Stokes lineshows a sharp decrease associated with an increase inthe pump power owing to pump depletion. In P205 -doped fibers, however, higher-order Raman-Stokeslines generated from the first Stokes peak (1.59 Mim)appear at 1.73 um owing to Si-O-Si bond vibrationand at 2.02 Mm owing to P=O bond vibration. Sincethe losses of the test fiber at 1.73 and 2.02 tim are largerthan 10 dB/km because of infrared absorptions, thehigher-order Raman process for the P205-doped fiberis suppressed. Therefore, the pump depletion issmall, and the saturation becomes weak.

The Raman power gain G of the test fiber can beevaluated with the equation8'9

G = exp, P PLeff - s(1)Aeff

Here Py is a polarization coefficient of SRS interaction,gp is a Raman power gain coefficient due to the P0Obond vibration, Pp is the pump power, Aeff is an effec-tive area of the SRS interaction, Leff is the effectivelength defined by Leff = [1 - exp(-apL)]/ap, L is thefiber length, and ap and a, are optical power losses ofthe fiber at the pump and Stokes wavelengths, respec-tively. For ordinary single-mode fibers, y is estimat-ed to be 1/2, taking into consideration the fact that thepolarizations of the pump and the Stokes waves getout of step. 9 The Raman gain coefficient gp can beestimated from the P205 concentration in the coreregion.5 For 17 mol % P2 05 , gp at 1.59Mim is calculatedto be 3.7 X 10-12 cm/W, which is half the Raman gaincoefficient of pure silica. The effective area Aeff iscalculated from the mode profile by using the overlapintegrall 1

Jo.? 2 2d1 - fp2d 2dS

Aeff J t2 dSJ 4J2dS(2)

Here, 4p and (b are the normalized amplitude distri-butions of the pump and Stokes waves, respectively.Assuming a Gaussian field distribution, Aeff can becalculated by using 7r(wp2 + w8

2 )/2, where 2wp and 2w,are the mode field diameters at the pump and Stokeswavelengths, respectively. Mode field diameters at1.3 and 1.55 Am, which are measured by the far-field-pattern method, are 7.4 and 9.7 Am, respectively.From Eq. (2), Aeff is calculated to be 5.86 X 10-7 cm2 .L, £Xp, and a, are 800 m, 2.8 dB/km, and 5.7 dB/km,respectively, and therefore Leff is calculated to be 625m. With these values, the Raman gain slope of thetest fiber is estimated to be 1.0 dB/W. The differencebetween the experiment and the estimation can beattributed to the polarization coefficient y. In thiscase, oy is assumed to be 1/2, which means that thepolarization states of the pump and the Stokes wavesare scrambled completely along the fiber. However,in the experiment the polarizations of the pump andthe Stokes waves are sometimes partially maintainedalong the fiber, and the SRS gain becomes larger thanestimated.

Wavelength dependence of the Raman power gain isshown in Fig. 3. The wavelength of the signal lightsource is varied by adjusting the driving current andthe temperature of the laser diode. The measurementis carried out over 12 nm around the gain maximumwavelength. The -3-dB Raman gain bandwidth at apump power of 9.5 W is 6 nm, which corresponds to abandwidth of 23.5 cm-1 . This gain bandwidth impliesthat the pulse that has a pulse width of 500 fsec can beamplified by using SRS of the P20 5-doped fiber. Thisresult suggests that optical solitons at a 1.5-Min band,with a pulse width of a subpicosecond, can be ampli-fied with a high signal-to-noise ratio (SNR) by pump-ing with a 1.3-,Mm-band light source. It is also possibleto use the P205-doped fiber as the gain medium of afiber Raman soliton laser.1" The -3-dB gain band-width of the P205-doped fiber is narrower than that ofthe GeO2-doped fiber whose Raman gain bandwidth is130-135 cm-.3 From Fig. 3, the Raman gain changewith respect to wavelength change, AG/AX, for a P205-doped fiber is 1.9 dB/nm, and that for a GeO 2 -dopedfiber is 0.3 dB/nm, which also means that the gaincurve for the P20 5-doped fiber is sharp.

Pump-power dependence of the amplified signaland spontaneous emission is shown in Fig. 4. The

15

m ~~~-3dB23.5 cm

10

< 5

o/ UMP POUWtH = f.tD

1.590 1.595 1.600WAVELENGTH (pm)

Fig. 3. Wavelength dependence of Raman gain.

668 OPTICS LETTERS / Vol. 13, No. 8 / August 1988

0EmcS -lo

w3' -200a.

: -30Q-

o -40

-10 10 20PUMP POWER (W)

Fig. 4. Pump-power dependence of theand the spontaneous emission.

In conclusion, for a pump power of 15 W at 1.32,Mm,a Raman power gain of 20 dB (net gain of 15.4 dB) hasbeen achieved at approximately 1.59 Mm by using aP20 5-doped silica single-mode fiber that has 17 mol %of P 20 5 in the core. The 3-dB gain bandwidth is 23.5cm'1, and the optical power ratio of the amplifiedsignal to the spontaneous emission is about 25 dB forthe first Stokes line. These characteristics are prom-ising for the amplification of optical solitons with ahigh SNR and for soliton lasers.

The authors would like to thank N. Uesugi, F.Ashiya, and H. Kimura for their encouragement.

amplified signalReferences

fiber input-signal peak power is about 10 MW, and theresolution of the grating monochromator is about 1nm. The optical power ratio between the amplifiedsignal and the spontaneous emission, which can bemeasured by switching the input signal, is 25.1 dB for a14-W pump. This value corresponds to a SNR (Isignal out/Ispontaneous emission) of 50.2 dB in an electrical circuitexpression. The SNR for the first Stokes line of ahighly GeO2 -doped fiber (1.42 Am) is 47.4 dB, wherethe pump wavelength is 1.34 Am.3 This means thatthe SNR for the P205-doped fiber is the same as thatfor the GeO2-doped fiber. However, the SNR for thesecond Stokes line of a GeO 2 -doped fiber (1.52 Mm) is18.4 dB for a pump wavelength of 1.34 Mm.3 From thepoint of view of 1.5-Mm-band amplification, low noiseamplification is more promising with a highly P20 5 -doped fiber using the 1.3 -Mm pump light source.However, since the loss of the P 20 5-doped fiber ismuch larger than that of the GeO2-doped fibers, it isimportant to reduce the optical loss of the P20 5-dopedfiber to less than 1 dB/km, so that more efficient am-plification with a lower pump power can be achieved.

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AMPLIFIEDSIGNAL

25.1dB

SPONTANEOUS