2008_s偏振中由布拉格体光栅宽范围可调谐ybkyw激光器

1、Appl. Phys. B 91, 8588 (2008)Applied Physics BDOI:10.1007/s00340-008-2937-2Tunable Yb:KYW laser using volume Bragg grating in s-polarizationLaser Physics, KTH Royal Institute of Technology, 10691 Stockholm, Swedenb. jacobssonj.e. hellstro mv. pasiskeviciusf. laurelldence of the reflectivity on the l

2、aser beam geometry, where high reflectivity can only be achieved if the angular bandwidth of the grating is wider than the angular distribution of the beam, given by the beam waist radius. The gratings angular bandwidth is given by its spectral bandwidth but additionally decreases with increasing in

3、cidence angle. Recently, rigorous solutions of the coupled-wave equations in a reflection Bragg grating at oblique incidence have been given and experimen- tally verified 12, 13, confirming the above simple physical picture.Received: 29 October 2007Published online: 19 February 2008 Springer-Verlag

4、2008ABSTRACT We report on a volume-Bragg-grating tunable Yb:KYW laser. By using s-polarized light on the grating, we obtain an output power of 3 W in a single beam, doubled com- pared to earlier p-polarization results. The laser tunability was from 996 nm to 1048 nm. Detailed measurements of the las

5、er spectrum revealed a bandwidth of 10 GHz.PACS 42.40.Eq; 42.55.Xi; 42.60.Fc2Experimental setup1IntroductionThe laser set-up was similar to the one described in 10. A 3 3 3 mm3 b-cut 5%Yb:KYW crystal was used. The two end facets were antireflection-coated while two op- posing side facets were covere

6、d by In-foil and pressed intoa water-cooled copper holder. Due to the highly anisotropic absorption in Yb:KYW, we used a polarized pump source with a polarization along the Nm dielectric axis of the crys- tal. The crystal was oriented in the holder in such a way that it provided the largest gain for

7、 the s-polarized beam. The pump was a non-fiber-coupled diode bar operating at 980 nm with a 2.4 nm (FWHM) spectral bandwidth. The pump beam was focused through a flat input coupler mirror to a spot with radiiIn volume Bragg gratings written in photo-thermo- refractive glass 1, it is possible to com

8、bine high peak reflec- tivities of more than 99% with narrow bandwidths of a few hundred picometers or less. Naturally, such properties are attractive for locking the wavelength of laser devices. This was first done for diode lasers 2, and subsequently for op- tical parametric oscillators 3, 4 and s

9、olid-state lasers 59. Using a retroreflector design, recently we also achieved wave- length tuning using a volume Bragg grating, both in an Yb laser 10 and an optical parametric oscillator 11. The laser medium that was used in the former work was Yb:KY(WO2)4 (Yb:KYW), which shows a wide emission spe

10、ctrum centered around 1025 nm, enabling a tuning range from 9971050 nm. In this laser a maximum power of 1.7W was obtained in a good qualitybeam.In this work we show how the output power of the laser can be doubled compared to 10, by using s-polarization (TE) on the grating instead of p-polarization

11、 (TM), due to increased grating reflectivity for oblique incidence. The dif- ferent incidence-angle sensitivity of the Bragg grating reflec- tivity for s- and p-polarizations stems from different boundary conditions for tangential and normal electric field compo- nents, and has been experimentally d

12、emonstrated in 12. In addition, to optimize a tunable laser that incorporates a ro- tating reflection Bragg grating with oblique incidence, one must take into account the sensitivity of the grating reflectiv- ity on the incidence angle and angular distribution of the laser cavity mode. Geometrical a

13、rguments can explain the depen- Fax: +46-8-5537-8216, E-mail: bjlaserphysics.kth.se(1/e2) of 80 100 m2 in the crystal. This focus was Gaussianalong the narrower direction and close to top-hat in the wider direction. The input coupler was designed for high-reflectivity (HR 99.9%) in the 10201200 nm w

14、avelength region. This reflectivity, Rin, decreases rapidly at shorter wavelengths, from Rin = 99.5% at 1010 nm to about Rin = 95% at 1000 nm,Rin = 75% at 995 nm and Rin 1% at the pump wavelengtharound 980 nm. The maximum pump power transmitted to the crystal was 19.1 W. Due to drift of the pump wav

15、elength with driving current, as well as a varying laser threshold with laser wavelength, the pump absorption under lasing conditions var- ied between approximately 70% and 90%.A reflective volume Bragg grating in a retroreflector de- sign was used to lock and tune the laser wavelength, as shown in

16、Fig. 1. This grating (Ondax) had a peak reflectivity for plane waves at normal incidence of 97% at 1063.5 nm and a FWHM bandwidth of 0.55 nm. The dielectric mirror of the retrore- flector was coated for HR between 1010 nm and 1050 nm at the corresponding incidence angles ranging between 63 andLasers

17、 and Optics86Applied Physics B Lasers and OpticsFIGURE 1 Laser setup77. In order to avoid the reduction in Bragg grating reflectiv- ity that occurs for narrow incident beams 12, the cavity wasdecreasing reflectivity of the input coupler. At longer wave- lengths, the reduction of gain in the laser cr

18、ystal eventually limits the tuning range in a given laser cavity. In Fig. 3, the output power dependence on the incident pump is shown fordesigned to have a 280 m waist at the grating. The beamwaist size was essentially limited by the aperture of available reflective Bragg grating. At the same time,

19、 the beam waist in the laser crystal has to be small to maintain low thresh- olds in the three-level system. Thus, we used a folded cavity with a 200 mm radius-of-curvature folding mirror, resulting insome selected wavelengths and R = 80%. The laser reacheda maximum output power of 3W at 1030 nm wit

20、h a threshold of 6.5W incident power and a slope efficiency versus inci- dent power of 25%. The efficiency in terms of absorbed power exceed 30%. The maximum tuning range was 9961048 nm with an R = 95% output coupler. At maximum power for alla 80 m waist inside the crystal. The total cavity length w

21、asapproximately 470 mm. Finally, the cavity was completed by a flat output coupler. The reflectivity, R, of the output coupler was varied between 80% and 90%.wavelengths, we measured a beam quality of M2 440 m, in good agree- ment with our experimental finding of a slight decrease in reflectivity fo

22、r our 280 m beam radius. The latter radius had to be used since the aperture of our grating did not al- low for larger waists. Second, some predictions about future lasers can be made. If we assume a grating of 99% reflec- tivity at 1064 nm and a beam waist of 300 m, the minimum FWHM bandwidth that

23、can be tolerated is 0.87 nm. However, for a grating with a FWHM bandwidth of 0.55 nm, the mini- mum beam waist is 480 m. Consequently, there will always be a trade-off between the practicality of the laser and the re- quired laser bandwidth, which must be balanced considering the intended applicatio

24、n.Increasing the grating reflectivity would also allow for tuning to longer wavelengths. Presently, the tuning range is limited to below 1050 nm due to the reduced laser gain at longer wavelengths, requiring lower cavity losses. As shown in 10, with a total output coupling of 3%, the laser could be

25、locked to a wavelength of 1063 nm, when the grating was used at normal incidence. This clearly indicates that a higher grating reflectivity should increase the tuning range further. Naturally, this also necessitates a grating with a longer peak wavelength at normalincidence.ence of the two peaks to

26、spatial hole-burning, allowing two non-neighboring longitudinal modes to lase simultaneously.By measuring the power loss through the grating and com- paring it with the output power, the grating reflectivity can be deduced. Figure 5 shows the reflectivities for s-polarization thus obtained for diffe

27、rent wavelengths. It also shows theor- etical predictions based on the model in 12, as well as the deduced reflectivities for p-polarization obtained in 10. It is clearly evident that s-polarization significantly reduces the losses through thegrating.Despite the advantage of s-polarization, Fig. 5 a

28、lso shows that there is still loss through the grating, which reduces the laser performance. To improve the system further, it would be necessary to optimize the grating parameters. A grating with a higher peak reflectivity would give higher output powers, while a grating with a narrower bandwidth m

29、ight reduce the laser bandwidth. As is shown in the following calculations, an optimization of the grating is a trade-off between a nar- row grating bandwidth, and the cavity mode size needed to accommodate for it. In practice, a too large cavity mode size would demand a very long and impractical la

30、ser cavity. As shown in 12, the effective reflectivity for oblique incidence of s-polarized light is a function of the internal incidence angle, , the beam waist radius, w, and the gratings spectral bandwidth, z, defined between the first minima on either side of the main reflection peak. As an appr

31、oximate rule given by (20) in 12, the reduction in reflectivity will be small if the following inequality is satisfied:4ConclusionsIn conclusion, we have demonstrated a route for optimizing the output power and efficiency of a tunable Yb:KYW laser which includes a rotating reflection Bragg grating a

32、s a tuning and wavelength-locking element. In par- ticular it was shown that s-polarized intracavity beams can give up to a doubling in the output power in a similar laser cavity, due to the reduced sensitivity of the grating reflectivity with the incidence angle for this polarization. The laser max

33、- imum output power in a single beam was 3 W, and the tuning range was 9961048 nm. Moreover, the spectral bandwidth was 10 GHz and the beam quality was essentially diffraction- limited for all pump powers. Further optimization of the tunable laser can be most practically achieved by employ- ing a gr

34、ating with a larger spectral bandwidth and a higher w z sin . 4 n0 (1)B BHere n0 denotes the average refractive index of the grating and B the Bragg wavelength at normal incidence. If we want to use the laser above 1000 nm wavelength, this corresponds to an angle 20, or sin 300 m nm.Next we look at

35、some examples of what is predicted.First, for the present system, we have a grating with 97% peak reflectivity at 1063.5 nm, with a FWHM bandwidth of0.55 nm, corresponding to z = 0.68 nm. The limitation for88Applied Physics B Lasers and Opticspeak reflectivity. Fortunately, the volume-Bragg grating tech- nology allows, to a large degree, independent design of the spectral bandwidth and the reflectivity obtained from the grat- ing. If required, single-longitudinal mode operation can be achieved either by employi