Single frequency ytterbium fiber laser andphosphosilicate Raman fiber laser
電子情報学専攻

植田研究室
黄聖鴻 (Huang Shenghong)2006 年 3 月

1. Single frequency ytterbium fiber laser system
As we know, there are many important applications for single frequency fiber lasers, such as
coherent optical communications, wavelength division multiplexing (WDM) system,high-resolution atomic and molecular spectroscopy and so on. Especially, single frequency 1064
nm laser can be used as a laser source in laser interferometric gravitational wave detector.
Narrow linewidth 1083 nm laser is an efficient laser source for helium pumping.
Usually a single frequency master oscillator power amplifier (MOPA) system can provide
high power, very narrow linewidth laser source for gravitational wave detector system, however,
the master laser is the solid-state laser. A Very complex optical coupling system must be
introduced between the master laser and the fiber amplifier, which leads to two problems: 1. thecoupling efficiency is not so high, laser power is lost partly in the coupling system; 2. there aresome optical reflection because of the complex optical elements, which is easy to produce laser
oscillation in the fiber amplifier. Such laser oscillation is very dangerous for the amplifier,particularly when it operates at high power.
In order to prevent the above problems, we proposed to construct single frequency fiber
laser as the master laser source. Based on this fiber laser, we can construct an all-fiber master
oscillator power amplifier (MOPA) system for laser interferometric gravitational wave detector.
In addition, narrow linewidth laser at 1083 nm can be used to pump helium gas. Helium
atoms absorb this laser and will become hyperpolarized helium gas (HPG) through radio
frequency (RF) discharge in the helium gas. This HPG can be applied in a very broad range of
disciplines: Material sciences, Chemistry, Biology, Medicine and so on. Among these potentialapplications, the opportunity to image the air spaces of human lungs has immediately risen of
considerable interest.
Usually there are several approaches to produce single frequency fiber laser: long
traveling-wave fiber ring lasers; short linear cavity distributed reflector (DBR) fiber lasers anddistributed feedback (DFB) fiber laser. Ring fiber lasers need many optical elements in thecavity, which make the system complex and expensive. DBR fiber laser can not produce highpower because of the short fiber cavity. As for the DFB fiber laser, it needs special grating
writing technology, this is not easy to be completed in ordinary laboratories.

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In our work, we proposed to produce single frequency fiber laser based on a very simple
linear cavity. In the laser cavity, we introduced loop mirror filter (LMF) to discriminate andselect laser longitudinal modes, and introduced polarization controller (PC) to suppress thespatial hole burning (SHB) effect, as shown in figure 1.

PC1A+

3dB coupler

S-Am

A_PC2
Saturable absorber

OCSaturable amplifierFig. 1 The laser oscillator is composed by two sections. The first one is the laser gain section(the right section). In this one, PC2 is used to control the wave polarization and eliminate theSHB effect. The second one is the saturable absorber section (the left section), it is a LMF (loopmirror with saturable absorber) act as a very narrow filter.

1.1

1.1
g0=-15g0=-17.5
g0=-201.0

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Intensity (arb.units)

Intensity (arb.units)

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L=0.5, g0=-70
L=1, g0=-35
L=2, g0=-20

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-200

0200

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Frequency (MHz)Frequency (MHz)

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Pf0=0.25,g0=-20
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Intensity (arb.units)

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Frequency (MHz)Fig. 2 The calculated profile of the dynamic Bragg grating, (a) Fiber length and the power ofmode pump is given, the frequency modulation become stronger when the absorption
coeffiecient become larger; (b) The power of pump mode is given, when the fiber length2

become longer, the bandwidth of the grating become narrow; (c) Fiber length is given, whenpump power increased, other channel become stronger.

LMF is the fiber loop mirror with saturable absorber (SA), in which counterpropagatingwaves interfere with each other, and the interference pattern can engender dynamic Bragg
grating. This grating is an absorption grating, which bandwidth is inverse proportional to thelength of SA. Usually the bandwidth range covers sub-MHz to GHz, which is much narrower
than the normal FBG’s. In addition, the center wavelength of the grating is determined by thelaser wavelength in it, in this way the wavelength can be changed easily by changing the laser
wavelength. The simulation result about the dynamic grating is shown in figure 2. For linear
laser cavity, it is easy to arouse spatial hole burning (SHB) effect in the gain fiber because of theinterference of the counterpropagating laser waves. As we know the SHB effect is deathfulobstacle for single-frequency generation. If we make the polarization states of thecounterpropagating waves perpendicular, we can destroy the interference of the laser waves, as
thus, SHB effect can be restrain to some extent.
According to the above advisement, we introduced LMF into the linear fiber cavity todiscriminate and select laser longitudinal modes, at the same time introduced polarization
controller (PC) in the gain fiber section to control the polarization states of thecounterpropagating laser waves. The total linear cavity is composed of LMF, ytterbium fiber as
the gain material and FBG at 1064 nm as the output coupler. Stable single-frequency 1064 nm
laser was generated by optimizing the polarization states of the laser waves in the cavity. We
measured and analyzed the laser frequency with a scanning Fabry-Perot interferometer, which
had a free spectral range (FSR) of 6 GHz and resolution of 150 kHz. The output power can goup to 14 mW when optimized the gain fiber length, the SA length and the reflectivity of theoutput coupler FBG, respectively. This laser can be wavelength tuned about 1 nm when tuning
the temperature of the output coupler FBG.
Based on the almost same experiment configuration, single-frequency 1083 nm laser wasproduced when using output coupler FBG at 1083 nm. The maximum output power was 14 mW
under the optimum fiber cavity configuration. We measured the linewidth of this laser usingdelayed self-heterodyning method with 25 km delay fiber. It showed that the laser linewidth was
as narrow as 2 kHz. The single frequency signal and heterodyning signal of SF 1083 nm laser isshown in figure 3. After the single-frequency laser source, we constructed ytterbium fiber
amplifier to amplify the laser signal. The gain fiber of the amplifier is optimized to be 5 m, and
it is backward pumped. In this way, the fiber laser oscillator and amplifier come into being3

all-fiber MOPA system. 177 mW single-frequency laser at 1083 nm was produced from thissystem eventually. The Schematic of the SF 1083 nm MOPA system is shown in figure 4, thecorresponding result is shown in figure 5.

9-15Heterodyne Signal (dB)FSRF-P=6GHz(a)6-0.63

3-0.66

0-0.69-3Voltage (V)Intensity (arb.units)-0.60-6-0.72-90.010 0.015 0.020 0.025 0.030 0.035
Time (s)(b)-30-45-60-75-9079.96 79.98 80.00 80.02 80.04
Frequency (MHz)Fig. 3 (a) Single frequency operation verified using a scanning Fabry-Perot interferometer witha free spectral range (FSR) of 6GHz and a resolution of 150KHz. (b) Lineshape of theheterodyne signal measured using delayed self-heterodyne method with 25 km delay fiber. Fromthe signal taken 3 dB down from the maximum value we estimate the FWHM of the laser
spectral linewidth is about 2 KHz.
976nm LD
Pump

Residual
Pump
PC1
SA3dB
WDM2coupler

Yb fiber

PC2

FBG

WDM1ISO

976nm LDPump

5m Yb fiber
amplifier material

1083nm laser
OutputFig. 4 Experiment setup of the single frequency ytterbium MOPA fiber laser,which is composed by two sections. The above section is the laser oscillator, thelow section is the fiber amplifier.

2. High power Raman fiber laser at 1178 nm
Laser sources in the yellow-orange spectra are particularly of interest for applications inmetrology, medicine, biology, display technology and so on. Especially, the 589 nm atomic
sodium-resonance radiation can produce a laser guide star (LGS) in astronomy.
Atmospheric turbulence blurs the light from distant stars and galaxies. In order to remove4this blurring when observe the astronomical object, scientists adopt adaptive optics that needs
reference object besides the astronomical object. Using a laser beam to create an artificial “star”
as the reference object is a wonderful idea. A yellow laser tuned to the wavelength of 589 nm
can excite sodium atoms at an altitude of ~100 km in the Earth's atmosphere to make a artificial“star”.

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Output power (mW)

Relative Intensity (dB)-40

SNR>25dB

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950

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18016014012010080
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40200050 100 150 200 250 300 350
Pump power (mW)

Wavelength (nm)Fig. 5 (Left) Optical spectrum of the master oscillator power amplifier (MOPA) fiber laser. The
signal-to-noise ratio (SNR) of the 1083nm laser is larger than 25 dB. (Right) 1083nm output
power versus pump power for given 4.25 mW signal input power.

At present, there are some methods to produce 589 nm yellow laser: Argon-ion pumped cw
dye-jet laser; Solid-state Nd:YAG lasers with sum-frequency generation(SFG); Nd fiber lasers
with SFG. In our experiment, we proposed to develop 589 nm laser based on Raman fiber laser
and frequency doubling technology. Firstly, we generate 1178 nm Raman laser based on
phosphosilicate fiber pumped by 1100 nm fiber laser. Next, 1178 nm laser is converted to 589
nm laser by frequency doubling technology.
As the other part of our work, we have generated high power 1178 nm laser using
phosphosilicate RFL, figure 6 shows the schematic of the experiment setup. Because thephosphosilicate fiber has very broad Raman spectrum, it is easy to transform the pump laser
wavelength to some special wavelength. We made use of the Raman shift of 601 cm-1 ofphosphosilicate fiber to produce 1178 nm laser, the pump source was a 20 W, 1100 nm
ytterbium doped double clad fiber laser. The maximum output power of 10.5 W is produced
from this Raman fiber laser. Figure 7 shows the experiment results. The laser wavelength isconverted to 589 nm light by frequency doubling technology, as shown in figure 8. This orange
yellow laser can be used as the laser source for laser guided star (LGS) in astronomy.

5Yb-dopedfiber laser
1100 nm
CW, 20WFBG11178nm
HRP doped fiber
fiberL=300m,(700m)

FBG2R=15%(30%, 50%)

Fig. 6 Experiment setup of Raman fiber laser: The fiber length is 300m (700m), the reflectivityof FBG2 is 15% (30%, 50%) at 1178nm, respectively.
120.6
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Output Power(W)Power(arb.units)

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11001120

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864
201180468

101214161820

Input Pump Power(W)Wavelength(nm)Fig. 7 (Left) Output spectrum from the Raman fiber laser. The Raman laser at 1178 nm, theresidual pump laser wavelength is 1100 nm. (Right) Evolution of the output powers for theresidual pump, the Stokes lights as the pump power is enhanced.

Prism

L2

Yb-doped
Fiber laser
1100nm

M2
L1FBG
1178nm
HRP doped fiber
300m

LBOin oven
M1

Fig. 8 Schematic of the experimental setup for frequency doubling of 1178 nm laser. Cavity isformed by FBG, M1. M2 is a dichroic mirror through which visible lights escape from thecavity.

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