2.3 Tunable Ytterbium doped fiber lasers
2.3.2 Results and discussions
The free-running performance of the present fiber laser was studied first. The external cavity only comprised a focusing lens and a high reflector without inserting the FP bandpass filter. The free-running performance was taken as a baseline to obtain the
1070 1080 1090 1100 1110 1120 1130
0 20 40 60 80 100
Transmittance (%)
λ (nm) max:95%@1102.4nm FWHM: 5.75nm
0 5 10 15 20 25 30 35
1030 1040 1050 1060 1070 1080 1090 1100 1110
Central wavelength (nm)
Angle of incidence (degree)
Fig. 2.7. Spectral transmittance of the thin-film FP 1.1-µm filter at normal incidence.
Fig. 2.8. Wavelength dependence on angle of incidence for the thin film FP 1.1-µm filter bandpass filter.
efficiency of the wavelength narrowing with the FP bandpass filter. Figure 2.9 shows the output powers with respect to the incident pump power in the free-running operation and the wavelength-narrowing operation at 1040nm, 1060nm, 1080nm, 1095nm, and 1100nm by changing the incident angle on the FP bandpass filter. In the free-running operation the maximum output power of was approximately 17 W at a pump power of 26 W. The slope and optical-to-optical efficiencies were 77% and 68%, respectively. The wavelength-narrowing operation had slightly lower slope efficiencies due to the insertion loss by the FP filter. The efficiency from the free running operation to the wavelength-narrowing operation can be generally greater than 90%.
The overall tuning range was shown in Fig. 2.10. The output powers were almost the same for the whole tuning range. Note that the upper wavelength was restrained by the peak transmission wavelength of the filter at normal incidence, whereas the lower wavelength was limited by the gain profile of the fiber. As we increased the AOI further, the Yb population required for lasing is higher that the gain at the gain peak becomes excessive, to the point where ASE at the gain peak starts to
Fig. 2.9. Output powers with respect to the incident pump power in the free-running operation and the wavelength-narrowing operation of tunable Yb fiber laser.
0 5 10 15 20 25 30
dominate the emission and a parasitic lasing occurred. It is noted that the gain profile is influence by the temperature of the core, the length of the fiber, and codoped dopants such as aluminates for refractive index controlling. Moreover the temperate-sensitive or length-sensitive property is due to the energy level of Yb ion as mentioned in 4.1. Sinha et al. at Stanford Univ. have produced a fiber laser emitting at 1150 nm by heating the fiber directly to red-shift the fluorescence of Yb fiber [26] and Dvoyrin et al. obtained an 1160-nm Yb fiber laser with a self-heating Yb fiber [27].
Oppositely a shorter-wavelength operation demands a shorter fiber or lower temperature of fiber itself.
The laser spectral performance was recorded by an OSA with maximum resolution of 0.1 nm (Advantest Q3489). As depicted in Fig. 2.11, the lasing linewidth is narrower with a linewidth of 0.36 nm than 5 nm of the FP filter, due to the normal linewidth narrowing occurring in the multi-pass amplification. Figure 2.12 shows the spectral bandwidth of the free-running operation and narrowband operation at various wavelengths. Linewidth of free-running operation was wider than 10 nm while the lasing linewidth were narrower than 1 nm for all tuning range at narrowband operation.
1040 1050 1060 1070 1080 1090 1100 1110
0 2 4 6 8 10 12 14 16 18
PP=23.1
16.6
10.1
Output power(W)
Wavelength(nm)
5.39
Fig. 2.10. Dependence of the output powers on the lasing central wavelength of tunable Yb fiber laser.
1094 1096 1098 1100 1102 1104 1106 1108 1110 0
20 40 60 80 100
Transmittance (%)
λ (nm)
Fig. 2.12. Experimental spectra for the free-running and wavelength-narrowing operations of tunable Yb fiber laser. The solid lines present narrowing spectra; the dot line presents the free-running operation.
Fig. 2.11. Comparison of the linewidth of the thin-film FP1.1-µm filter and that of the narrow-linewidth Yb fiber laser.
1020 1030 1040 1050 1060 1070 1080 1090 1100 1110 0.0
0.2 0.4 0.6 0.8 1.0
Intensity (a.u.)
Wavelength (nm)
2.4 Tunable Er/Yb codoped fiber lasers
In this section the FP filter is a standard DWDM filter with a bandwidth of 50 GHz.
We demonstrated that an efficient tunable Er/Yb doped fiber laser can be simply achieved with a 50 GHz FP bandpass filter. The efficiency from a free running lasing with a bandwidth of >5 nm to a narrow linewidth lasing with a linewidth < 0.15 nm can exceed 96%. With tilting the incidence angle, the lasing wavelength can be tuned from 1536 nm to 1564 nm with a tuning range up to 28 nm.
2.4.1 Experimental setup
Figure 2.13 depicts a schematic diagram of the experimental setup. The laser cavity comprised a 7-m double-clad Er/Yb codoped fiber and an external feedback cavity.
The absorption coefficient of the gain fiber was lower than that of the Yb doped fiber, approximately 3dB/m at 976 nm. The fiber had a core diameter of 25 µm and a cladding diameter of 400 µm with corresponding numerical apertures (NA) of 0.07 and 0.46. The fiber was coiled with a 12-cm diameter to increase the losses for the higher order transverse modes and strengthen the single-spatial-mode operation.
The external cavity was composed of an aspherical lens with 8-mm focal length, a thin-film FP bandpass filter for selecting and narrowing the emission wavelength, and a rear dielectric mirror with high reflectivity at 1530-1570 nm. The whole Fig. 2.13. Schematic configuration of the tunable and narrowband Er/Yb doped fiber laser.
HR @ 1530~1600 nm FP filter
HT@976 nm HR@1530~1600 nm
Laser output Fiber-coupled LD @976 nm
Er/Yb codoped double-clad fiber; 300/25 μm
HR @ 1530~1600 nm FP filter
HT@976 nm HR@1530~1600 nm
Laser output Fiber-coupled LD @976 nm
Er/Yb codoped double-clad fiber; 300/25 μm
cavity was bounded by the rear mirror and a perpendicularly cleaved fiber end facet as the output end with ~4% Fresnel reflection. The fiber end facet adjacent to the external cavity was not angled cleaved to suppress broadband feedback. The pump source and pump scheme were identical to those employed in the above section.
The spectral transmittance of the thin-film FP filter at normal incidence is shown in Fig. 2.14. It can be seen that the transmission bandwidth is approximately 50 GHz and the central wavelength is around 1564.2 nm with a transmission up to 98%.
Note that the characteristics of the present bandpass filter are appropriate to the demand of WDM applications.
The thin-film FP bandpass filter used in our experiments had a clear aperture of 1.4 mm × 1.4 mm and was 0.5mm thick. The central wavelength can shift lower at oblique incidence. Therefore, the bandpass filter was mounted rigidly on a precision rotation stage to control the incident angle for wavelength tuning. In terms of the incident angle, the wavelength of peak transmittance is given by equation (1) with the parameters λo=1.56 µm, no=1, and neff is determined with the curve-fitting to the experimental measurement. As shown in Fig. 2.15, Eq. (1) can be used to express
1556 1558 1560 1562 1564 1566 1568 1570
0 20 40 60 80 100
FWHM: 0.4nm max:96%@1564nm
Transmission (%)
Wavelength (nm)
Fig. 2.14. Spectral transmittance of the thin-film FP 1.1-µm filter at normal incidence.
the wavelength dependence on angle of incidence up to 15o with neff =1.37.
2.4.2 Results and discussions
The free-running performance of the present fiber laser was studied first. For this investigation, the external cavity only comprised a focusing lens and a high reflector without inserting the FP bandpass filter. The free-running performance provides the baseline for evaluating the efficiency of the wavelength narrowing with a FP bandpass filter. Figure 2.16 shows the output powers with respect to the incident pump power in the free-running operation and the wavelength-narrowing operation at normal incidence. In the free-running operation the maximum output power was approximately 3.7 W at a pump power of 18.5 W with a slope efficiency of 22%;
while the slope efficiency was degraded to 7.9% at higher pump power owing to lasing of Yb3+ ion mainly. The wavelength-narrowing operation had nearly the same performance at the output power. The corresponding output power and slope efficiency at 1564nm, 1552nm, and 1536 nm are shown in the Fig as well. The highest efficiency from the free running operation to the wavelength-narrowing
0 3 6 9 12 15
1535 1540 1545 1550 1555 1560 1565
Central wavelength (nm)
Angle of incidence (degree)
Fig. 2.15. Wavelength dependence on angle of incidence for the thin film FP 1.56-µm filter bandpass filter.
operation can be generally greater than 92%. This result confirms the insertion loss of the thin film FP to be very low, comparable to the performance of VBGs in fiber lasers [chapter 1, 32-33].
With changing the incident angle on the FP bandpass filter, the lasing central wavelength can be tuned from 1564.2 nm to 1536 nm, corresponding to the incident angle varying from 0° to 15°. The overall tuning range was shown in Fig. 2.17. The output powers were almost the same for the whole tuning range. Note that the upper wavelength was restrained by the peak transmission wavelength of the filter at normal incidence; whereas the lower wavelength was limited by the gain profile of the fiber.
0 3 6 9 12 15 18 21 24 27
0 1 2 3 4 5
Output power (W)
Incident pump power (W) freerunning (η=22%, 7.9%)
1564nm (η=20%, 6.3%) 1552nm (η=21%, 5.3%) 1536nm (η=20%, 5.4%)
Fig. 2.16. Output powers with respect to the incident pump power in the free-running operation and the wavelength-narrowing operation of tunable Er/Yb fiber laser.
1535 1540 1545 1550 1555 1560 1565
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
15 W
9.5 W
Output power (W)
Wavelength (nm)
Pp=18.5 W
Fig. 2.17. Dependence of the output powers on the lasing central wavelength of tunable Er/Yb fiber laser.
1563.7 1563.8 1563.9 1564.0 1564.1 1564.2 1564.3 1564.4 1564.5 1564.6 0
20 40 60 80 100
Transmission (%)
Wavelength (nm)
Fig. 2.18. Comparison of the linewidth of the thin-film FP 1.56-µm filter and that of the narrow-linewidth Er/Yb fiber laser.
The lasing spectra were measured by employing an optical spectra analyzer (Advantest Q8347) with a resolution of 0.007 nm. As depicted in Fig. 2.18, the spectral bandwidth of the narrowband operation was narrower with a linewidth <0.15 nm than 0.4 nm of the FP filter. Figure 2.19 shows the linewidth of the free-running mode and that of narrowband mode over all tunable range. Linewidth of free-running operation was wider than 5 nm while the lasing linewidth were narrower than 0.2 nm for all tuning range at narrowband operation.
2.5 Conclusion
An efficient tunable narrow-linewidth Yb and Er/Yb fiber lasers have been obtained with an all-dielectric FP bandpass filter as a wavelength selector. In Yb fiber experiment, the lasing spectrum can be reduced from a free-running wide band of 10 nm to a narrow band of <1 nm with a thin-film FP bandpass filter. The insertion loss introduced by the thin-film FP filter is so low that the overall output powers remain
>93% of the free-running output power. With tilting the incident angle to shift the passband curve, the lasing wavelength can be tuned from 1040nm to 1100 nm with a
1530 1535 1540 1545 1550 1555 1560 1565 1570
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1
Intensity (log scale)
λ (nm)
Fig. 2.19. Experimental spectra for the free-running and wavelength-narrowing operations of tunable Er/Yb fiber laser. The solid lines present narrowing spectra; the dot line presents the free-running operation.
tuning range greater than 60 nm. In the Er/Yb fiber laser experiments, an efficient tunable narrow-linewidth Er/Yb fiber laser has been obtained with an all-dielectric FP bandpass filter as a wavelength selector. The lasing spectrum can be reduced from a free-running wide band of >5 nm to a narrow band of <0.15 nm with a thin-film FP bandpass filter. More importantly, the insertion loss introduced by the thin-film FP filter is so low that the overall output powers remain >92% of the free-running output power. With tilting the incident angle to shift the passband curve, the lasing wavelength can be tuned from 1536nm to 1564.2 nm with a tuning range greater than 28 nm.
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Chapter 3 Passively Q-switched Ytterbium Fiber Lasers by use of Cr
4+: YAG crystals
3.1 Introduction to passively Q-switching
Passively Q-switching (PQS) is an important technique providing a compact method for generating pulse with duration of ns-scale normally or ps-scale for microchip type PQS lasers. PQS is modulated by a passive material such as organic dye, doped monolithic crystal, or semiconductor saturable absorber. These saturable absorbers can absorb light of their optical transition line and the absorption can be described simply by the equation:
( )
1 /
o o
i s
E E E
α = α
+ , (3.1) where αo is the small-signal absorption coefficient, E is the incident energy i density, and E is a saturation fluence s
(
e s a)
modehν A
sat σ +σ
E = , (3.2)
where σgs is the absorption cross section for the optical transition. The absorption is intensity dependent and saturable absorbers become more transparent when they absorb light. Figure 4.1 shows the transmission characteristics. The transmission is initially invariant and has a jump when E close toi E . At higher incident energy s density saturable absorber is saturated or bleached and is nearly a constant. PQS is based on the behavior of the absorption which provides a high loss initially in a resonant to prevent resonating and to store population inversion in gain medium; as saturable absorber is pumped continuously by the fluorescence of gain medium, it is transparent finally and the gain exceeds the loss which results in a pulse output.
There are a number of publications dealing with modeling of PQS lasers [1-5]. Here I will introduce and derive several important equations based on Ref. [1,4]
The parameters such as pulse energy, pulse peak power, and second threshold criterion will be obtained by including the parameters of intracavity focusing and the ESA effect into the rate equation. The coupled rate equations used here were based on that photon is axially uniformly distributed and the saturable absorber recovers completely.
(3.3)
(3.4)
(3.5) (3.6)
where φ is the effective intracavity photon density; Rp is the volumetric pump rate; n is population density of gain medium; ls is length of saturable absorber; τf and τs are the recovery time of upper laser level of gain medium and excited-state of saturable
p
Fig. 3.1. Nonlinear transmission of a saturable absorber versus incident energy density normalized to its saturation energy density.
absorber, respectively; A and As are effective area in the gain medium and saturable absorber respectively; ngs, nes, and nso are ground, excited state, and total population densities respectively; σgs andσes are ground and excited state cross sections in the saturable absorber respectively; R is reflectivity of the output coupler; γ is inversion reduction factor, γ=1 and γ=2 correspond to respectively, four-level and three-level systems [1]; tr is round-trip transit time of light in the cavity optical length. Figure
3.2 shows the simulated results of development of passively Q-switched laser pulses.
It is obviously seen that the gain exceeds the loss at some points, at which the photon number does not raise rapidly. The reason is that in passively Q-switched lasers there are two criterions for pulse-formation. 1st is the lasing criterion, i.e. gain surpasses loss. Satisfying the condition means there is probability to radiate photons, however it needs more condition to generate pulsed output, the condition is the so-called 2nd criterion. The details will be discussed later. Figure 3.3 shows the detail of Fig. 3.2 for a single pulse generation. It can be observed that the pulse peak occurs at the point that the gain equals the loss. After the point the loss exceeds gain again and continues to increase the population inversion of gain medium to output another pulse in the next cycle. The simulated results provides the view of the development of Q-switched pulse, however it is necessary to study this topic more analytically to give insight into the mechanism.
0 20 40 60 80 100 120 140
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
Normalized parameters
Rountrip time (a.u.) gain
loss photon
Fig. 3.2. The development of passively Q-switched laser pulses.
Firstly the terms Rp and n/τf in (3.4) and nes/τs in (3.5) are dismissed due to the build-up time of the Q-switched laser pulse is generally quite short compared with them. Now that (3.4) and (3.5) could be written as
53.36 53.38 53.40 53.42 53.44
0.0
53.36 53.38 53.40 53.42 53.44
0.0
Fig. 3.3. The development of a single passively Q-switched pulse. The maximum value of photon density occurs at the gain equals the loss.
and ni is the initial population inversion density in the gain medium, which can be obtained by setting the right-hand side of (3.3) to zero and assume that n(ngs=ns0)=ni. The value of initial population inversion density ni can be expressed as
(3.11)
Here the relation of the initial transmission of saturable absorber To =e−σgs so sn l is used. The value of pulse energy can be obtained with the aid of (3.7) as
(3.12)
The value of nf can be obtained as follows:
1. Dividing (3.3) by (3.7) and replacing (3.9) into the result gives
(3.13)
2. Integrating (3.13) gives
and then setting ψ(n=nf)=0 [ψ(n=ni)=0 is a trivial solution gives
2 2
Eqs (3.15) can be solved numerically to obtain nf.
nφmax is the maximum photon density of fast saturable absorber where α is infinity.
Besides it is necessary to derive a criterion for Q-switching as depicted in Ref. [4].
The author derived a modified 2nd threshold criterion for Q-switching by including the influence of intracavity focusing and the ESA effect into the rate-equation analysis.
The threshold was obtained by setting
2
be positive assuring a growth curve
for photon intensity will turn upward.
for photon intensity will turn upward.