June 1986 / Vol. 11, No. 6 / OPTICS LETTERS 383

Efficient single-pass Raman generation in a GeO2 optical fiberand its application to measurement of chromatic dispersion

H. Takahashi, J. Chang, K. Nakamura, I. Sugimoto, T. Takabayashi, and A. Oyobe

Central Research Laboratory, The Furukawa Electric Company, 2-9-15 Futaba, Shinagawa-Ku, Tokyo 142, Japan

Y. Fujii

Electrotechnical Laboratory, 1-1-4 Umezono, Sakura, Niihari, Ibaraki 305, Japan

Received February 6, 1986; accepted March 31, 1986

To illustrate nonlinear-optical applications of germanium dioxide (GeO2) glass optical fibers, we have demonstrated

efficient single-pass Raman generation in a GeO2 fiber. We also measured the chromatic dispersion of high-silica

glass (SiO2 ) single-mode optical fibers by using a GeO2 fiber Raman laser.

Introduction

A GeO2 optical fiber has been developed as a candidatefor an infrared optical fiber that transmits wave-lengths longer than 2 ,gm. These fibers were reportedto have an attenuation of 4 dB/km at 2 ptm whenprepared by the vapor-phase axial-deposition meth-od.'

In addition to its prospects as an infrared opticalfiber, the GeO2 optical fiber has been investigated as anonlinear-optics research medium because the Ra-man-scattering cross section of GeO2 glass seems to benearly 1 order of magnitude greater than that of silicaglass.2 For nonlinear-optical applications, a single-mode fiber is more useful than a multimode fiber, butsingle-mode GeO2 optical fiber is not yet available.Such fiber is still under development in our laborato-ry. We therefore made two preliminary experimentsusing multimode GeO2 optical fibers. The first ex-periment was single-pass Raman generation, and thesecond experiment was measurement of chromaticdispersion in Sio 2 single-mode optical fibers.

Single-Pass Raman Generation in a GeO2 Fiber

Although an attenuation of 4 dB/km at 2-,gm wave-length was obtained as the lowest loss value of a GeO2optical fiber,' the loss of the fiber used in this experi-ment was less than 50 dB/km between 1 and 1.5 gnm,except for the OH peak around 1.4 ,m. This was thebest fiber available for the experiment. The core andcladding diameters of the fiber were 93 and 185 gum,respectively. The fiber length was 28 m. The experi-mental setup for single-pass Raman generation wasessentially the same as that previously employed.3 ANd:YAG laser, mode locked at 100 MHz and Qswitched at a 2-kHz repetition frequency, was used asa pump source at a wavelength of 1.064 ,im. The pulsewidth (FWHM) was 300 psec, and the duration of thepulse train was 220 nsec.

At 0.3-W total transmitted power, high-order

Stokes light (up to sixth order) was observed on aphosphorous screen. Figure 1 shows a typical spec-trum of the output light. The data are not correctedfor the wavelength dependence of the PbS detectorand the monochromator. The Stokes shift was 424cm-1, and the broad tail that always appears in asingle-pass Raman generation in SiO2 fibers was notobserved. These results are consistent with a singlepeak in the Raman-gain spectrum of vitreous GeO2.2

The pump-power dependence of the transmittedpump light and that of the first-, second-, and third-order Stokes light are shown in Fig. 2. The abscissa isthe average power of the total transmitted light, whichis fairly close to the input pump power because of theshort length of the fiber. The first-order Stokes lightappeared at average powers above 120 mW. Pump-power depletion is seen as a deviation from the lineardependence above 150 mW, where second-orderStokes light begins to build up. The critical pumppower for the GeO2 fiber was 9 kW for the 93-pm core-

~i

Li00.

1.0

ir

01: 1.0 1.15

ilWAVELENGTH (pm)

I I I I~~~~~~~~~~~.

WAVELENGTH (mm)

Fig. 1. Transmitted light spectrum of single-pass Ramangeneration in a GeO2 fiber. The fiber length was 28 m, andthe total transmitted power was 0.3 W.

0146-9592/86/060383-03$2.00/0 © 1986, Optical Society of America

1 .2 1 .4 1 .6

384 OPTICS LETTERS / Vol. 11, No. 6 / June 1986

30

I.-0Z;

a:

20

10

00 " 100 200 300 350

AVERAGE OUTPUT POWER (mW)

Fig. 2. Pump-power dependence of the transmitted pumplight and the first-, second-, and third-Stokes light. Thefiber length was 28 m.

diameter, 28-m-long fiber, whereas the critical powerfor SiO2 fiber with the same fiber parameters and aRaman gain of 0.92 X 10-11 cm/W is estimated to beabout 80 kW.

Given a single-mode GeO2 optical fiber with 2-gmcore diameter and a loss of less than 20 dB/km, only 1W of input pump power is required for single-passRaman generation. This power level is readily ob-tained from a pulsed laser diode. Fiber Raman lasers,excited by laser diodes, seem promising as passive andactive devices for the characterization of fibers and fordirect signal amplification through Raman gain.

Measurement of Chromatic Dispersion in SiO2Fibers

As another application, the chromatic dispersion ofhigh-silica glass single-mode optical fibers was mea-sured by using Stokes light generated in GeO2 fiber.The loss of the GeO2 optical fiber used in this experi-ment was about 20 dB/km in the 1.0-1.7-gm region(except for the OH peak around 1.4 gim). A fiberabout 90 m long was used in the experiment; the nu-merical aperture of the fiber was 0.1. The core andcladding diameters of the fiber were 70 and 150 gim,respectively. The pump was a Nd:YAG laser (X =1.064 gin). Unlike the pump used in the first experi-ment, the laser was acousto-optically mode locked at160 MHz and simultaneously Q switched at 1 kHz.With approximately 5 kW of peak pump power in thegenerating fiber, stimulated Stokes light of up to theeighth order was observed. Figure 3 shows the spec-tral emission obtained in this experiment. Instead ofthe continuous, broad spectrum that always appearsin SiO2 fiber Raman lasers, each Stokes line generatedwith the GeO2 fiber Raman laser was discrete andnarrow (a few nanometers' spectral width), just asseen in the first experiment.

The experimental arrangement for measuring the

chromatic dispersion of single-mode optical fibers isshown in Fig. 4. As an example, the transmissiontimes of first through seventh Stokes pulses along a3.96-km-long SiO2 single-mode optical fiber are shownin Fig. 5. For comparison, results obtained from aSiO2 fiber Raman laser are also shown in Fig. 5. Thetwo sets of results are in good agreement. The zero-dispersion wavelength was found to be 1.331 gim. Be-cause the spectrum of the GeO2 fiber Raman laserswas so narrow and discrete, the monochromator (in-dispensable in a measuring system with an SiO 2 fiberRaman laser) was replaced by a simple grating to sepa-

vUI.--

LxAL

It-03

0.

40 -.30 -

201-

6-5-4-

3-2-1-

0-1.0

GeO2 fiber

Length =90m

N.A. =0.1

A. A

1.1 1.2 1.3 1.4 1.5 1.6 1.7

WAVELENGTH (pm)

Fig. 3. Spectrum of transmission of single-pass Ramangeneration in a GeO2 fiber. The fiber length was 90 m.

Test fiberGeO2 fiber Amplifier

Nd:Y A G S~~~~~~~~~~~~amplingLaser .,. oscilloscope

Mode Q-switched Simple grating A locked - Trgge

generator

Microcomputer

Fig. 4. Experimental arrangement for measuring chromat-ic dispersion of single-mode optical fibers. APD, avalanchephotodiode.

I

0sInc)

-J

0Li

1.0 1.1 1.2 1.3 1.4

WAVELENGTH (pm)

1.5 1.6

Fig. 5. Relative transmission delay versus wavelength for a3.96-km single-mode optical fiber.

I - o GeO2 fiber Raman laser

0 1 st * SiO2 fiber Raman laser9

8 ~~~~~~~~~7th7

62nd

4-

3 - \3rd 6th2 4

n

11

June 1986 / Vol. 11, No. 6 / OPTICS LETTERS 385

rate Stokes orders. Moreover, we could feed all theStokes pulses into the test fiber simultaneously andresolve the transmission time of each order throughsignal processing.

The dynamic range of the measurement systemcould therefore easily be improved by more than 10dB. Furthermore, the power of each Stokes-orderlight was concentrated and distributed with almostequal wavelength intervals over the 1.0-1.7-,gm range,so the zero-dispersion wavelength of optical fibers inthe 1.3- or 1.55-gim region could be found rapidly andaccurately. This system can therefore be applied tothe measurement of chromatic dispersion in a long-span optical link. If we could obtain a single-modeGeO2 fiber, Raman generation could be stimulatedfrom a laser diode. Measurement would then be sim-pler and more compact.

Summary

We have performed nonlinear-optical experiments onGeO2 optical fibers. Efficient single-pass Raman gen-eration in a GeO2 fiber was observed, and the criticalpeak power for the 93-gm core-diameter, 28-m-long

GeO2 fiber was measured at 9 kW, which is nine timessmaller than that for SiO2 fiber. Second, by usingStokes light as the light source, the chromatic disper-sion of single-mode SiO2 optical fibers was measured,and the measurement system has been found usefuland convenient.

These were preliminary experiments. If we obtaina GeO2 single-mode fiber, which is now under develop-ment, we shall be able to obtain more accurate resultsat greater convenience.

This research was supported in part by the Agencyof Industrial Trade and Industry of Japan in theframework of the National Research and Develop-ment project titled "Optical Measurement and Con-trol Systems."

References

1. H. Takahashi and I. Sugimoto, IEEE J. Lightwave Tech-nol. LT-2, 613 (1984)

2. F. L. Galeener, J. C. Mikkelsen, Jr., R. H. Geils, and W. J.Mosby, Appl. Phys. Lett. 32, 34 (1978).

3. L. G. Cohen and C. Lin, IEEE J. Quantum Electron. QE-14, 855 (1978).