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Optics Express

Optics Express

  • Editor: Michael Duncan
  • Vol. 13, Iss. 7 — Apr. 4, 2005
  • pp: 2611–2616
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Self-pulsed fiber Raman master oscillator power amplifiers

Yan Feng and Ken-ichi Ueda  »View Author Affiliations


Optics Express, Vol. 13, Issue 7, pp. 2611-2616 (2005)
http://dx.doi.org/10.1364/OPEX.13.002611


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Abstract

We report self-pulsed operation of fiber Raman master oscillator power amplifiers, in which the amplifier and oscillator are pumped by one pump source successively. The pulse period is one or half of the round-trip time of the oscillator, depending on the optical length of the amplifier. A simple model is constructed to explain the observations qualitatively.

© 2005 Optical Society of America

1. Introduction

Fiber Raman devices are very flexible in wavelength as gain is available at arbitrary wavelengths with right pump source, and can be widely tunable for broad Raman gain spectrum in silica fiber. Therefore, fiber Raman devices are very attractive for a variety of applications. Efficient high power fiber Raman lasers have been realized [1

1. M. Prabhu, N.S. Kim, and K. Ueda, “Simultaneous two-color CW Raman fiber laser with maximum output power of 1.05 W/1239 nm and 0.95 W/1484 nm using phosphosilicate fiber,” Opt. Commun. 182, 305–309 (2000). [CrossRef]

3

3. Shenghong Huang, Yan Feng, Akira Shirakawa, and Ken-ichi Ueda, “Generation of 10.5 W, 1178 nm laser based on phosphosilicate Raman fiber laser,” Jpn. J. Appl. Phys. 42, L 1439–L 1441 (2003). [CrossRef]

] and commercialized [4

4. IPG Photonics, “RLM Series: 1 to 10 Watts Raman Fiber Lasers”, http://www.ipgphotonics.com/html/101_raman_fiber_lasers.cfm

]. Here we report highly efficient pulsed operation of fiber Raman master oscillator power amplifiers (MOPA) without any active switching device.

The recent development of fiber Raman laser benefits mostly from the advancement in high power Yb doped fiber lasers, so all of them operate at near infrared wavelength regime. Frequency conversion of fiber Raman lasers to visible regime could open up potential in tunable visible fiber-based laser source, or visible lasers at wavelength not obtainable by other means [5

5. Yan Feng, Shenghong Huang, Akira Shirakawa, and Ken-ichi Ueda, “Multiple-color cw visible lasers by frequency sum-mixing in a cascading Raman fiber laser,” Opt. Express 12, 1843–1847 (2004), http://www.opticsexpress.org/abstract.cfm?URI=OPEX-12-9-1843 [CrossRef] [PubMed]

]. In particular, frequency doubling of 1178nm fiber Raman laser or amplifier to 589nm is very interesting for the potential application in laser guide star adaptive optics [6

6. Joshua C. Bienfang, Craig A. Denman, Brent W. Grime, Paul D. Hillman, Gerald T. Moore, and John M. Telle, “20 W of continuous-wave sodium D2 resonance radiation from sum-frequency generation with injection-locked lasers,” Opt. Lett. 28, 2219–2221 (2003). [CrossRef] [PubMed]

8

8. Peter W. Milonni, Heidi Fearn, John M. Telle, and Robert Q. Fugate, “Theory of continuous-wave excitation of the sodium beacon,” J. Opt. Soc. Am. A 16, 2555–2566 (1999). [CrossRef]

]. In this context, pulsed operation of fiber Raman laser source is interesting for better frequency conversion efficiency.

In this paper, fiber Raman MOPAs, which are pumped by one laser source successively (firstly amplifier, then oscillator), are investigated. Because the oscillator and amplifier share the same pump, they are coupled to each other; the output of fiber Raman MOPAs can be self-pulsed in certain configurations. In the experiments the pulse periods are one or half of the round-trip time of the oscillator, depending on the optical length of the amplifier.

2. Experimental setup

Experimental setup is shown in Fig.1. The laser system consists of an oscillator and an amplifier, which are spliced together directly. A double clad Yb fiber laser at 1100nm is used as a pump source. Through a WDM (1100nm/1178nm), it pumps the amplifier and oscillator successively. The laser at 1178nm from the oscillator propagates leftward, gets amplified, and finally emits out through the WDM. In the experiments, several configurations with different outcoupling of the oscillator and different length of the fibers used in oscillator and amplifier are investigated. The fibers used in experiments are HI1060 single mode fiber or phosphosilicate single mode fiber.

Fig. 1. Schematic of experimental setup. FBG1 is a partially reflective fiber Bragg grating (15%, 30%, 50%), and FBG2 is a highly reflective (99%) fiber Bragg grating.

In such kind of system one may imagine following processes. When the oscillator is on, the pump laser, which reaches the oscillator, would be depleted because part of its energy is consumed by amplifying the laser. Then the pump laser reaching the oscillator could be lower than the laser threshold in certain case, so the oscillator could be off. When the oscillator is off, the pump laser would not be depleted. So the pump laser reaching the oscillator could be larger than the laser threshold again.

Mathematically, such kind of processes could lead to a fix-point or limiting-cycle solution depending on parameters, which means cw or repetitive pulsed operation, respectively. In experiments we see both type of operations indeed.

3. Results and discussion

In one of the experiments, the fiber used in the amplifier is a phosphosilicate fiber, and in the oscillator a HI1060 fiber, the reflectivity of the FBG1 is 15%. The laser system reaches threshold at a pump power of 3.7W, and remains cw until 4.4W. At 4.5W, the laser output becomes sinusoidal wave suddenly. Increasing the pump power further, the sinusoidal wave develops to pulses. Fig.2 shows typical waveforms at pump power of 4.5W, 6.7W, and 13.8W, respectively. The period of pulses is about 4.28µs, which doesn’t change with the pump power.

To understand the origin of the period, we measure the optical fiber length. Simple fiber Raman lasers with each fiber as gain medium are constructed, the beat frequencies of their cw output are measured. For the phosphosilicate fiber and HI1060 fiber used in above MOPA experiment, the beat frequencies are found to be 323.3kHz and 116.7kHz, respectively. Corresponding round trip times are 3.09µs and 8.57µs, and corresponding fiber lengths are about 320m and 890m, respectively (the refractive indexes of both fibers are considered as 1.445). One may notice the period 4.28µs is half of the round-trip time of the oscillator, 8.57µs.

Fig. 2. Waveforms of laser output at pump power of a) 13.8W, b) 6.7W, and c) 4.5W, respectively. The fiber used in the amplifier is a 320m phosphosilicate, in the oscillator an 890m HI1060 fiber, and the reflectivity of FBG1 is 15%.

Experiments with other configurations are also investigated. When FBG1 is changed to one with reflectivity 30%, the laser threshold decreases to 2.4W, the pulsing threshold increases to 12W, and the pulse period is still 4.28µs. When the reflectivity of FBG1 is 50%, the laser threshold decreases to 1.8W; the laser is cw in full pump range. In the case that the fiber used in the amplifier is a 710m phosphosilicate fiber, in the oscillator the same HI1060 fiber, and FBG1 15%, the laser threshold is 4.1W, the pulsing threshold is 5W, and the pulse period becomes 8.57µs. In the case that the fiber used in the amplifier is the 890m HI1060 fiber, in the oscillator the 320m phosphosilicate fiber, and FBG1 30%, the laser threshold is 4.9W, the pulsing threshold is 7.8W, and the pulse period becomes 3.1µs.

Figure 3 shows typical waveforms from three configurations at pump power near 11W. From these experiments we see that the periods of laser dynamics are one or half of the round trip time of the oscillator, depending on the length of the amplifier.

A simple model is constructed to explain the observations. The classical treatment of the stimulated Raman scattering process in optical fibers yields a system of first-order coupled partial differential equations [9

9. B. Burgoyne, N. Godbout, and S. Lacroix, “Transient regime in a nth-order cascaded CW Raman fiber laser,” Opt. Express 12, 1019–1024 (2004), http://www.opticsexpress.org/abstract.cfm?URI=OPEX-12-6-1019 [CrossRef] [PubMed]

,10

10. M. Karásek and M. Menif, “Channel Addition/Removal Response in Raman Fiber Amplifiers: Modeling and Experimentation,” J. Lightwave Technol. 20, 1680–1687 (2002). [CrossRef]

].

Pz+1vPt=grυpυRP(I++I+2hυPB)αPP
I+z+1νI+t=grP(I++hυRB)αRI+
Iz1vIt=grP(I+hυRB)+αRI,
(1)

where P, I+, and I - stands for the power of the pump laser, the forward propagating Stokes emission, and the backward propagating Stokes emission, respectively; v is velocity of light in fiber; gr, αP, and αR are Raman gain coefficient, loss coefficient for pump light and loss coefficient for Raman signal. 2PB and RB stands for the contribution of spontaneous Raman emission, where h is Plank constant, υP and υR are the frequencies of the pump and Stokes emission, B is a Boltzmann factor [9

9. B. Burgoyne, N. Godbout, and S. Lacroix, “Transient regime in a nth-order cascaded CW Raman fiber laser,” Opt. Express 12, 1019–1024 (2004), http://www.opticsexpress.org/abstract.cfm?URI=OPEX-12-6-1019 [CrossRef] [PubMed]

], which is very close to 1 here. The boundary conditions are

P(0)=P0
I+(0)=0
Il(La)=Ir(La)·(1R1)+Il+(La)·R1
Ir+(La)=Ir(La)·R1+Il+(La)·(1R1)
I(La+L)=I+(La+L)·R2
(2)

where L and La are the fiber lengths in the oscillator and amplifier; 0, La, and La+L in bracket represent for the positions of the output end, FBG1, and FBG2, respectively; The subscript l and r stand for left and right side of the point; R 1 and R 2 are the reflectivity of the FBG1 and FBG2, respectively; P 0 is input pump power.

Since the observed dynamics are periodic, we try a general periodic solution of following form

I+(z,t)=m=0CI+m(z)cos(mk(zvt)+ϕI+m)
I(z,t)=m=0CIm(z)cos(mk(z+vt)+ϕIm).
(3)

Substitute to the boundary condition, after some algebra we find k must be π/L. So laser output is periodic with a period of 2L/v in general. However, in the experiments we have observed not only 2L/v, but also half of the round-trip time, L/v. This means all coefficients, CI+m and CI-m, with odd m equal to zero in that case. We try numerical simulation to explain this.

Fig. 3. Waveforms of laser output in three configurations with pump power near 11W (the curves are offsetted for clear presentation). From top to bottom, a) the fiber used in the amplifier is a 710m phosphosilicate fiber, in the oscillator an 890m HI1060 fiber, and the reflectivity of FBG1 is 15%; b) in the amplifier a 320m phosphosilicate fiber, in the oscillator an 890m HI1060 fiber, and FBG1 15%; c) in the amplifier an 890m HI1060 fiber, in the oscillator a 320m phosphosilicate fiber, and FBG1 30%.

Fig. 4. top) calculated waveforms for two configurations at pump power of 12W: the reflectivity of FBG1 is 15%, in oscillator an 890m HI1060 fiber, in amplifier (I) 320m and (II) 710m phosphosilicate fiber, respectively, which correspond to the experimental configurations; bottom) calculated waveforms for the configuration (I) at pump power of 1) 6W, 2) 12W, and 3) 18W, respectively.

However, the rising edge of the calculated pulses is not as sharp as in experiments. The peak power is smaller than in experiments. The duty cycle of calculated pulses is generally larger than that observed in experiments. Possibly, there are some other effects, which assist the pulsed operation, not included in our simple classical model.

Fig. 5. the laser output power as a function of pump power for three configurations with different reflectivity of FBG1, R1, length of phosphosilicate fiber in amplifier, La, and same fiber in oscillator, which is an 890m HI1060 fiber.

4. Summary

In summary, we report observation of self-pulsed fiber Raman master oscillator power amplifiers pumped by one laser source successively. The pulsing behavior results from the coupling of the oscillator and amplifier for sharing the same pump source. A simple model is used to explain the observations qualitatively. Efficiency of such a MOPA scheme is as high as of an oscillator.

Acknowledgments

This work is supported by the 21st Century COE program of Ministry of Education, Science and Culture of Japan. Yan Feng’s homepage is http://yanfeng.org/work.

References and links:

1.

M. Prabhu, N.S. Kim, and K. Ueda, “Simultaneous two-color CW Raman fiber laser with maximum output power of 1.05 W/1239 nm and 0.95 W/1484 nm using phosphosilicate fiber,” Opt. Commun. 182, 305–309 (2000). [CrossRef]

2.

S. A. Skubchenko, M. Y. Vyatkin, and D. V. Gapontsev, “High-Power CW Linearly Polarized All-Fiber Raman Laser,” IEEE Photon. Technol. Lett. 16, 1014–1016 (2004). [CrossRef]

3.

Shenghong Huang, Yan Feng, Akira Shirakawa, and Ken-ichi Ueda, “Generation of 10.5 W, 1178 nm laser based on phosphosilicate Raman fiber laser,” Jpn. J. Appl. Phys. 42, L 1439–L 1441 (2003). [CrossRef]

4.

IPG Photonics, “RLM Series: 1 to 10 Watts Raman Fiber Lasers”, http://www.ipgphotonics.com/html/101_raman_fiber_lasers.cfm

5.

Yan Feng, Shenghong Huang, Akira Shirakawa, and Ken-ichi Ueda, “Multiple-color cw visible lasers by frequency sum-mixing in a cascading Raman fiber laser,” Opt. Express 12, 1843–1847 (2004), http://www.opticsexpress.org/abstract.cfm?URI=OPEX-12-9-1843 [CrossRef] [PubMed]

6.

Joshua C. Bienfang, Craig A. Denman, Brent W. Grime, Paul D. Hillman, Gerald T. Moore, and John M. Telle, “20 W of continuous-wave sodium D2 resonance radiation from sum-frequency generation with injection-locked lasers,” Opt. Lett. 28, 2219–2221 (2003). [CrossRef] [PubMed]

7.

Yan Feng, Shenghong Huang, and Akira Shirakawa et al., “589nm light source based on Raman fiber laser,” Jpn. J. Appl. Phys. 43, L722–L704 (2004). [CrossRef]

8.

Peter W. Milonni, Heidi Fearn, John M. Telle, and Robert Q. Fugate, “Theory of continuous-wave excitation of the sodium beacon,” J. Opt. Soc. Am. A 16, 2555–2566 (1999). [CrossRef]

9.

B. Burgoyne, N. Godbout, and S. Lacroix, “Transient regime in a nth-order cascaded CW Raman fiber laser,” Opt. Express 12, 1019–1024 (2004), http://www.opticsexpress.org/abstract.cfm?URI=OPEX-12-6-1019 [CrossRef] [PubMed]

10.

M. Karásek and M. Menif, “Channel Addition/Removal Response in Raman Fiber Amplifiers: Modeling and Experimentation,” J. Lightwave Technol. 20, 1680–1687 (2002). [CrossRef]

OCIS Codes
(140.3280) Lasers and laser optics : Laser amplifiers
(140.3510) Lasers and laser optics : Lasers, fiber
(140.3550) Lasers and laser optics : Lasers, Raman

ToC Category:
Research Papers

History
Original Manuscript: March 4, 2005
Revised Manuscript: March 22, 2005
Published: April 4, 2005

Citation
Yan Feng and Ken-ichi Ueda, "Self-pulsed fiber Raman master oscillator power amplifiers," Opt. Express 13, 2611-2616 (2005)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-13-7-2611


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References

  1. M. Prabhu, N.S. Kim, and K. Ueda, "Simultaneous two-color CW Raman fiber laser with maximum output power of 1.05 W/1239 nm and 0.95 W/1484 nm using phosphosilicate fiber," Opt. Commun. 182, 305-309 (2000) [CrossRef]
  2. S. A. Skubchenko, M. Y. Vyatkin, and D. V. Gapontsev, "High-Power CW Linearly Polarized All-Fiber Raman Laser," IEEE Photon. Technol. Lett. 16, 1014-1016 (2004) [CrossRef]
  3. Shenghong Huang, Yan Feng, Akira Shirakawa, and Ken-ichi Ueda, "Generation of 10.5 W, 1178 nm laser based on phosphosilicate Raman fiber laser," Jpn. J. Appl. Phys. 42 , L 1439-L 1441 (2003) [CrossRef]
  4. IPG Photonics, "RLM Series: 1 to 10 Watts Raman Fiber Lasers", <a href="http://www.ipgphotonics.com/html/101_raman_fiber_lasers.cfm">http://www.ipgphotonics.com/html/101_raman_fiber_lasers.cfm</a>
  5. Yan Feng, Shenghong Huang, Akira Shirakawa, and Ken-ichi Ueda, "Multiple-color cw visible lasers by frequency sum-mixing in a cascading Raman fiber laser," Opt. Express 12, 1843-1847 (2004), <a href="http://www.opticsexpress.org/abstract.cfm?URI=OPEX-12-9-1843">http://www.opticsexpress.org/abstract.cfm?URI=OPEX-12-9-1843</a> [CrossRef] [PubMed]
  6. Joshua C. Bienfang, Craig A. Denman, Brent W. Grime, Paul D. Hillman, Gerald T. Moore, and John M. Telle, "20 W of continuous-wave sodium D2 resonance radiation from sum-frequency generation with injection-locked lasers," Opt. Lett. 28, 2219-2221 (2003) [CrossRef] [PubMed]
  7. Yan Feng, Shenghong Huang, Akira Shirakawa et al., "589nm light source based on Raman fiber laser," Jpn. J. Appl. Phys. 43, L722-L704 (2004) [CrossRef]
  8. Peter W. Milonni, Heidi Fearn, John M. Telle, and Robert Q. Fugate, "Theory of continuous-wave excitation of the sodium beacon," J. Opt. Soc. Am. A 16, 2555-2566 (1999) [CrossRef]
  9. B. Burgoyne, N. Godbout, and S. Lacroix, "Transient regime in a nth-order cascaded CW Raman fiber laser," Opt. Express 12, 1019-1024 (2004), <a href="http://www.opticsexpress.org/abstract.cfm?URI=OPEX-12-6-1019">http://www.opticsexpress.org/abstract.cfm?URI=OPEX-12-6-1019</a> [CrossRef] [PubMed]
  10. M. Karásek and M. Menif, "Channel Addition/Removal Response in Raman Fiber Amplifiers: Modeling and Experimentation," J. Lightwave Technol. 20, 1680-1687 (2002) [CrossRef]

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