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Applied Physics B

, 124:13 | Cite as

A discretely tunable dual-wavelength multi-watt Yb:CALGO laser

  • Sujith Manjooran
  • Pavel Loiko
  • Arkady Major
Article

Abstract

A discretely tunable dual-wavelength diode-pumped Yb:CALGO laser using a single birefringent filter (BRF) plate which covered a wavelength range of approximately 1020–1070 nm was demonstrated. A detailed study was conducted for BRF plates with thickness of 0.5, 2, 4 and 6 mm using different output couplers. This simple design was capable of delivering multi-watt dual-wavelength output power and the frequency offset discretely varied from approximately 1.3 to 12.5 THz. The maximum dual-wavelength output power was 4.1 W using a 6-mm-thick BRF plate with 5% output coupler.

1 Introduction

Dual-wavelength lasers have been attractive for a wide range of applications such as laser spectroscopy, optical coherence tomography, THz generation and LIDAR instrumentation [1, 2]. Most of the earlier works on dual-wavelength lasers were either in low-power (mW) regime or used complex designs. Examples of the complex designs are multi-ethalon configuration [2], Bragg gratings [3], moving mirror technique [4] or two adjacent pump spots in one crystal [5]. However, a dual-wavelength operation can be achieved in a simple way by introducing a single BRF plate into a standard laser cavity. BRF plates are wavelength-dependent periodic loss elements and their free spectral range is mainly determined by their thickness [6]. Earlier experiments using BRF plates were carried out in the low-power regime. A combination of two BRF plates was tested by Trevino-Palacios et al. [7] in Ti:sapphire laser. A frequency offset of 1 THz was achieved using BRF plates with thickness of 2.082 and 33.315 mm. Unfortunately, the output power was limited to less than 150 mW. Medium power (850 mW) dual-wavelength operation of alexandrite laser using a single BRF plate was demonstrated in Ref. [8]. At the same time, Demirbas et al. [9] used a tilted-optic axis (45°) BRF plate to generate dual-wavelength in Cr:Nd:GSGG and Cr:LiSAF lasers with < 370 mW of output power. The mode-locked operation of a dual-wavelength Cr:Nd:GSGG laser was also demonstrated by the same group with an average output power of 45 mW [10].

Dual-wavelength operation based on rare earth-doped (Yb3+, Nd3+) gain media are of particular interest because of their ability to generate multi-watt output power owing to high-emission cross sections and relatively low heat loading that can be offered by diode laser pumping [11, 12, 13, 14]. Nd3+-doped materials are also well suited for multi-color operations [9, 15, 16, 17]. However, crystalline Nd3+-doped materials are mostly limited by their available gain bandwidth [18]. In addition to that, the multiple sharp fluorescence peaks arising from transitions between the fixed electronic manifolds or transitions between the sublevels of the same manifold make it difficult to achieve dual-wavelength tunability.

In this context, Yb3+-doped materials are favourable because of their substantially broader gain bandwidths. In the past decade, this was successfully utilized for efficient generation of ultrashort pulses [19, 20, 21, 22, 23, 24, 25, 26] that are needed for a large number of applications [27, 28, 29, 30]. At the same time multi-watt dual-wavelength outputs were also realized. Using an Yb:YCOB crystal, 3.14 W at 1084.3 nm and 3.45 W at 1061.3 nm [31] were produced using a custom-made optics. However, major limitations of the design were that it was working only with one special output coupler and the output wavelengths or their separations were not tunable. Yb3+-doped monoclinic double tungstates (MDTs), KY(WO4)2 and KGd(WO4)2 (shortly KYW and KGW) are also attractive candidates in terms of wavelength tunability, because of their broad gain bandwidth [32]. The research works based on dual-wavelength MDT lasers are the following. Brenier et al. [33] generated dual-wavelength radiation using two-volume Bragg gratings inside the laser cavity. Zhao et al. [34] generated a multi-watt dual-wavelength radiation with different polarizations based on anisotropic thermal lensing [35, 36]. Recently, Akbari et al. [37] demonstrated a multi-watt dual-wavelength output with single polarization using an Yb:KGW crystal and a single BRF plate. In both cases the free spectral range was limited to approximately 20 nm (the gain bandwidth of Yb:KYW and Yb:KGW is 16 and 20 nm, respectively [32]) which, therefore, puts constrains on the achievable dual-wavelength frequency offset range. As a result of this limited bandwidth, thin BRF plates (e.g., 2 mm thick or thinner) could not produce dual-wavelength operation [37]. Another disadvantage of the MDTs is their moderate thermal conductivity (~ 3.0 Wm−1 K−1 [38]) which makes these crystals more prone to damage in multi-watt operation regime.

On the other hand, Yb3+-doped tetragonal calcium gadolinium aluminate CaGdAlO4 (CALGO) has several advantages when compared with MDTs. High-power dual-wavelength operation with Yb:CALGO laser gain medium is an attractive option because of its broad gain bandwidth (~ 80 nm) and high thermal conductivity (6.9 Wm−1 K−1) [39]. This crystal is optically uniaxial [40, 41] and it has one of the highest thermal conductivities among other Yb3+-doped crystals, which makes it suitable for high-power operation [42, 43, 44]. Taking advantage of such unique properties, this paper reports on a broad range of dual-wavelength operation by adjusting the thickness of a single BRF plate placed in the laser cavity. The demonstrated free spectral range discretely varied between 4.7 nm and 47.1 nm depending on the plate thickness and operated in a multi-watt regime.

2 Theory and experiment

The gain medium was a 5-mm-thick a-cut 2 at.% Yb:CALGO crystal (Castech). It was pumped at 979 nm with a fiber-coupled InGaAs diode laser (NA = 0.12 and 105 µm core diameter) from IPG Photonics. The pump radiation was unpolarised. The crystal was water cooled and was wrapped in indium foil of 125 µm thickness for better heat removal. Both of the end faces of the Yb:CALGO crystal were anti-reflection (AR) coated for the pump and laser wavelength to reduce intracavity losses. The pump beam was imaged into a 315 µm spot diameter in the crystal with the help of two lenses of 150 and 50 mm focal lengths. The maximum pump power absorbed under the non-lasing condition was 14.9 W at an incident pump power of 18.4 W. The design of a five-mirror laser cavity took into account thermal lensing [45] and is shown in Fig. 1.

Fig. 1

Schematic of a laser cavity. The distances used were: D 1 = 1000 mm, D 2 + D 3 = 408 mm, D 4 = 300 mm, D 5 = 550 mm. Radii of curvature of the curved mirrors: M 1 = − 750 mm, M 2 = − 500 mm. The angles were ξ 1 = ξ 2 = 4°. OC output coupler. Inset shows the crystal dimensions and orientation

For dual-wavelength operation, a BRF plate was inserted into one of the arms of the laser cavity (see Fig. 1). The dual-wavelength spectra were recorded for BRF plates of thickness 2, 4 and 6 mm using different output couplers. A single-wavelength tuning was also tested using a 0.5-mm-thick plate. Each BRF used a quartz plate (Castech) schematic of which is presented in Fig. 2. The BRF plate was placed at the Brewster angle to minimize the insertion loss and to act as a polarization-sensitive loss element. This allowed only the horizontal σ-polarization to oscillate inside the laser cavity.

Fig. 2

Schematic of a quartz BRF plate. The optical axis (\(\vec {c}\)) was on the surface of the BRF plate. The angle between the projection of the incident beam on the surface of the BRF plate and the optical axis (\(\vec {c}\)) is called the tuning angle (α). The angle of incidence was the Brewster angle (ϕ B). The BRF plate was rotated around its surface normal (\(\vec {N}\)). γ is the angle between the optical axis (\(\vec {c}\)) and the direction of the internal (refracted) ray

A simple simulation of transmission windows based on the average wavelength of 1047 nm and a 2-mm-thick BRF plate is shown in Fig. 3. As can be seen, if a laser gain medium with a broad spectral bandwidth is used, it is possible to introduce ‎a wavelength-dependent loss such that only two distinct wavelengths benefit from the effective gain. In other words, the net-gain of the two particular wavelengths can be equalized by the combined action of the transmission window of the BRF plate and the gain curve of the crystal. The periodic transmission and the corresponding peak wavelength separation (Δλ) are inversely proportional to the BRF plate thickness (d). Therefore, the broad gain bandwidth of Yb:CALGO can accommodate two (or more) transmission windows of a BRF [37].

Fig. 3

Calculated power transmission of a 2-mm-thick BRF (left y-axis). Also shown is the gain cross section (σ g) spectrum of Yb:CALGO for σ-polarization [44] and population inversion ratio β of 0.16

Details of the working principle of a BRF plate were thoroughly explained and modeled by Naganuma et al. [6]. More details and simulations can be seen in [46, 47, 48, 49, 50, 51]. The angle notation used for simulations in this work is similar to Ref [6]. Quartz is a positive uniaxial crystal with two principal refractive indices, namely ordinary (n o) and extraordinary (n e), and has a trigonal structure (α-SiO2). The n o and n e refractive indices of the quartz plate were calculated using the Sellmeier equation [52]. Average refractive index (n o(λ) + n e(λ))/2 was used for Brewster angle ϕ B(λ) calculation.

The separation of the transmitted wavelengths can be determined using Eq. (1) [46]:
$$\varDelta \lambda =\frac{{{\lambda ^2}\sin ~{\phi _{\text{B}}}}}{{\left( {{n_{\text{e}}} - {n_{\text{o}}}} \right)d~(1 - {{\cos }^2}\alpha ~{{\cos }^{2~}}{\phi _{\text{B}}})}}.$$
(1)
The free spectral range (FSR) of the BRF can be estimated following Eq. (2) [53], where c is the velocity of light and d is the thickness of the BRF plate:
$$\varDelta {\upsilon _{{\text{FSR}}}}=\frac{{c~\sin ~{\phi _{\text{B}}}}}{{\left( {{n_{\text{e}}} - {n_{\text{o}}}} \right)d~(1 - {{\cos }^2}\alpha ~{{\cos }^{2~}}{\phi _{\text{B}}})}}.$$
(2)

The transmission of power can be adjusted by rotating the BRF plate around its surface normal (\(\vec {N}\)) by changing the tuning (or rotation) angle α defined in Fig. 2.

Figure 4 emphasizes the fact that the tuning angle has a relatively small effect on FSR. The reason for this behavior is that the optical axis is located on the surface plane of the BRF plate. However, the variation was more significant for thinner crystals as shown in Fig. 4. The average wavelength for all calculations was taken as 1047 nm. The variation in average wavelength also has little effect on Δλ. The estimated values of FSR for different BRF plate thickness are shown in Table 1.

Fig. 4

Variation of FSR with respect to BRF thickness for different tuning angles (α = 20°, 40° and 60°)

Table 1

Calculated values of Δλ and FSR with respect to the BRF plate thickness for tuning angle α = 40°

BRF plate (mm)

n o

n e

n e – n o

Δν FSR (THz)

Δλ (nm)

2

1.5343

1.5431

0.00875

17.4

63.6

4

8.7

31.8

6

5.8

21.2

3 Results and discussion

Initially, a free-running operation was studied without using any intracavity BRF plates. In this regime, the output wavelength entirely depended on the output coupler. This can be explained by the quasi-three-level nature of laser transitions which leads to the wavelength-dependent gain and loss [54]. The results are displayed in Fig. 5. The best performance was obtained for 10% OC.

Fig. 5

Output power versus absorbed pump power for various output couplers (5, 10 and 15%). λ c is the central wavelength

After the measurements in the free-running regime, a BRF plate was introduced into one of the cavity arms (see Fig. 1). In this case, beyond a certain level of pump power, the spectrum splits into two lines, giving rise to a dual-wavelength output. A typical example of the dual-wavelength result is shown in Fig. 6.

Fig. 6

Dual-wavelength measurements using 6-mm-thick BRF plate. Output power versus absorbed pump power: transition from a single-wavelength operation to dual-wavelength operation with 10% OC; measured dual-wavelength spectrum is shown in the inset

It was possible to switch between a single-wavelength and dual-wavelength operation by varying the pump power. Figure 6 shows the switching between the single- and dual-wavelength regimes just by reducing the pump power. In this example, a combination of 6-mm-thick plate and 10% OC was used for the measurements. The dual-wavelength operation switched back to stable single-wavelength regime when the absorbed pump power was lower than 8.1 W. Therefore, at low pump powers the laser operated in single wavelength and at high pump powers the laser operated in a dual-wavelength regime. In the range of pump powers between the 8.1 and 9.8 W the laser was switching between the single- and dual-wavelength regimes. The linewidths of both oscillating wavelengths for 6-mm-thick plate were measured to be no more than 0.07 nm wide at half maximum. This measurement, however, was limited by the resolution of the used spectrometer. The recorded linewidth for a 2-mm-thick plate was broadened to 0.3 nm, as expected [8, 46]. By careful rotation of the plate the power ratio of the two generated wavelengths could also be adjusted.

The dual-wavelength experiment started with a 5% OC and 6-mm-thick BRF plate (Fig. 7a). It was more likely to get dual-wavelength regime using a thick BRF plate and a low-transmission output coupler. The laser with the 6-mm-thick plate generated an output power of 4.1 W at an absorbed pump power of 14.9 W. The wavelength separation in this case was 16.6 nm with two peaks at 1040.7 and 1057.3 nm. Next, the 6-mm-thick BRF plate was replaced by a 4-mm-thick one and the output power decreased to 2.95 W. In this case, in accordance with the theory, the dual-wavelength separation increased to 28.7 nm with peak wavelengths at 1033.3 and 1062.0 nm as shown in Fig. 7a. As the BRF plate was replaced with a 2-mm-thick one, the power further reduced to 1.41 W while the wavelength separation further increased to 47.1 nm with peak wavelengths of 1021.4 and 1068.5 nm. In all cases, dual-wavelength operation could be achieved at specific rotation angles of the BRFs. For example, for the 2-mm-thick BRF plate with 5% OC, the dual-wavelengths were observed around discrete angles of 56° (FSR = 46.9 nm) and 126° (FSR = 47.1 nm). In all cases the polarization of both emission lines was the same (σ) and horizontal, i.e., set by the Brewster angle of incidence of BRFs.

Fig. 7

a Spectra of dual-wavelength operation for 5% OC using 2-, 4- and 6-mm-thick BRF plates. b Spectra of dual-wavelength operation for 7.5% OC using 2-, 4- and 6-mm-thick BRF plates. c Spectra of dual-wavelength operation for 10% OC using 2-, 4- and 6-mm-thick BRF plates

Similar experiments were repeated for 7.5 and 10% OC as shown in Fig. 7b and c, respectively. The wavelength separation was increasing when thinner plates were used as in the previous case. However, the peak wavelengths were slightly different in all the cases. The mean emission wavelength was around 1047 nm.

The results are summarized in Fig. 8. All of them were taken at a fixed absorbed pump power of 14.9 W. As can be seen, there is a relation between the maximum dual-wavelength output power and the plate thickness: the maximum dual-wavelength output power scales with the plate thickness. This can be explained by the shape of the gain spectrum, i.e., the smaller FSR of a plate generates two wavelengths that are closer to the peak of the gain and vice versa.

Fig. 8

Summary of dual-wavelength operation results for 2-, 4- and 6-mm-thick BRF plates using different output couplers

The power ratio and stability of the two generated wavelengths was highly dependent on the intracavity power due to gain competition [37]. The power ratio fluctuations were more pronounced for thinner BRF plates and output couplers with higher transmission. For example, the power fluctuations observed for a thin BRF plate (2 mm) with higher output couplers (i.e., 7.5 and 10% OC) were up to 20% from the average value (i.e., at 1:1 power ratio), whereas the power fluctuations for a thick BRF plate (6 mm) with lower output coupler (i.e., 5% OC) were less than 5%. A possible explanation for this behavior is that for a 6-mm-thick BRF the observed dual-wavelength emission occurs in the relatively flat region of the gain spectrum closer to its peak, whereas for a 2-mm-thick BRF, the two emission lines are separated much wider and oscillate at the steeper and weaker regions of the gain spectrum. Therefore, even a small variation in loss can induce a sudden fluctuation in the output power levels.

The experiments were also conducted for 15 and 20% OC. However, as expected from narrowing of the Yb:CALGO gain spectrum for larger inversion ratios [54], it did not work for thinner BRF plates with large FSRs as shown in Fig. 8. The difference in FSR for a given BRF plate could be a result of the combined effect of gain dynamics and the tuning angle. We believe that the choice of output coupler is the more influential factor for these small (2–3 nm) variations, because the output coupler determines the gain dynamics [54]. The tuning angle was only slightly adjusted to achieve a 1:1 power ratio.

The entire sets of experiments were also conducted for a very thin BRF plate of 0.5-mm thickness. This thin BRF plate is suitable for single-wavelength tuning because of large FSR. Single-wavelength tuning results using different OCs are shown in Table 2. In this case, the maximum power was observed in the middle of the tuning range. Surprisingly, we also got dual-wavelength operation that does not comply well with the theoretical values of FSR. More than one dual-wavelength combinations were possible with 0.5-mm-thick plate by simple power adjustments (Table 2). A possible explanation of this behavior is that the fairly broad transmission window of the 0.5-mm-thick BRF plate in combination with a fairly flat gain of Yb:CALGO could support two narrowly separated peaks as shown in Fig. 9. In contrast to the regular operation of BRF where each generated wavelength is caused by one of its low-loss transmission windows (see Fig. 3), in this case dual-wavelength regime was supported by a single transmission peak due to the mentioned peculiarities of the gain spectrum.

Table 2

Single-wavelength tuning results using 0.5-mm-thick BRF plate

OC

Single-wavelength tuning (output power)

Single-wavelength tuning range

Observed dual-wavelength emission lines

Δλ (Δν)

Dual-wavelength power (W)

5%

1043.3 nm (3.0 W)

1050.3 nm (4.1 W)

1056.0 nm (0.9 W)

1043.3–1056.0 nm (12.7 nm)

(1) 1043.9 nm + 1048.8 nm

(2) 1039.6 nm + 1048.9 nm

4.9 nm (1.34 THz)

9.3 nm (2.56 THz)

3.3

2.1

7.5%

1043.0 nm (3.3 W)

1047.5 nm (3.9 W)

1049.5 nm (2.5 W)

1043.3–1049.5 nm (6.2 nm)

(1) 1044.7 nm + 1049.5 nm

4.8 nm (1.31 THz)

2.6

10%

1043.0 nm (3.4 W)

1047.7 nm (3.7 W)

1052.0 nm (1.6 W)

1043.0–1052.7 nm (9.7 nm)

(1) 1043.8 nm + 1048.5 nm

4.7 nm (1.29 THz)

3.3

15%

1039.3 nm (2.0 W)

1045.0 nm (2.4 W)

1050.0 nm (1.1 W)

1039.3–1050.0 nm (10.7 nm)

(1) 1039.7 nm + 1044.5 nm

4.8 nm (1.33 THz)

1.9

20%

1039.6 nm (0.3 W)

1041.8 nm (0.5 W)

1042.3 nm (0.4 W)

1039.6–1042.3 nm (2.7 nm)

None

None

None

Fig. 9

Calculated power transmission of the 0.5-mm-thick BRF plate and measured dual-wavelength spectrum. Also shown is the gain cross-sectional spectrum of Yb:CALGO for σ-polarization and population inversion ratio β of 0.13

A brief comparison of similar works in the multi-watt regime using Yb:KGW laser is as follows: the maximum output power obtained for thermal lens-driven simultaneous dual-wavelength operation of Yb:KGW laser was 4.6 W at a pump power of 26.8 W [34]. The FSR options were very limited in this experiment and polarization-switching was observed. Another work on Yb:KGW utilizing the option of a single BRF plate produced a lower output power of 3.4 W at a pump power of 27.9 W [37]. The maximum possible dual-wavelength separation using BRF plate thickness was limited by the gain bandwidth of Yb:KGW. In the present paper, a higher output power level was reported (4.1 W) with a much lower pump power of 18.4 W. In addition to the significantly wider discrete tunability of the possible frequency offset, the optical-to-optical efficiency (η o-o) of this work was 22.3%, which is much higher than the η o-o of the above-mentioned Yb:KGW lasers (17.2%) [34] and 12.2% [37]. This points out that Yb:CALGO is a more attractive choice than Yb:KGW for multi-watt dual-wavelength operation with widely tunable frequency offset.

4 Conclusion

A diode-pumped dual-wavelength Yb:CALGO laser was demonstrated based on the use of a single intracavity BRF plate. Four BRF plates of thicknesses 0.5, 2, 4 and 6 mm were tested in the experiments to produce discretely tunable separation of the generated wavelengths. Average wavelength separation for 2-, 4- and 6-mm-thick plates were 45.6 nm (Δν = 12.5 THz), 27.9 nm (Δν = 7.7 THz) and 16.6 nm (Δν = 4.4 THz), respectively. The maximum output power of 4.1 W in dual-wavelength regime was observed for 6-mm-thick BRF plate using an optimized output coupler transmission of 5%. For the 0.5-mm-thick BRF plate, a single-wavelength output and occasionally a closely separated dual-wavelength regimes were observed. The latter one can be explained by the broad transmission window of the BRF and the relatively flat gain spectrum of Yb:CALGO. We believe that the developed dual-wavelength Yb:CALGO laser with multi-watt output power is a suitable and cost-effective candidate for dual-wavelength mode-locked lasers [55, 56, 57] and generation of THz radiation [58]. At the same time such lasers can be attractive as a pump source for second harmonic generation or optical parametric oscillators in the visible and near-IR spectral ranges [59, 60, 61, 62, 63].

Notes

Acknowledgements

The authors would like to acknowledge funding of this project provided by the Natural Science and Engineering Research Council of Canada (NSERC), Western Economic Diversification Canada, and the University of Manitoba. P. Loiko acknowledges financial support from the Government of the Russian Federation (Grant No. 074-U01) through ITMO Post-Doctoral Fellowship scheme.

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Copyright information

© Springer-Verlag GmbH Germany, part of Springer Nature 2017

Authors and Affiliations

  1. 1.Department of Electrical and Computer EngineeringUniversity of ManitobaWinnipegCanada
  2. 2.ITMO UniversitySt. PetersburgRussia

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