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Abstract
Two compact mid-infrared microchip lasers at 2717 and 2740 nm have been demonstrated using a Er:Y2O3 ceramic as laser gain medium with thickness of 800 μm, for the first time to our knowledge. Under a 976-nm diode laser pumping, the 2717 nm microchip laser with linewidth of about 0.16 nm is achieved with a maximum output power of 234.8 mW and slope efficiency of about 10.9%. The laser beam quality expressed by M2 factor is measured to be about 1.23 and 1.45 in x and y directions. A single wavelength at 2740 nm with linewidth of about 0.15 nm is also achieved with maximum output power of 102 mW and slope efficiency of about 4.9%. Beam quality of the 2740 nm laser is found to be about 1.15 and 1.26 in x and y directions. Using a mechanical chopper to modulate the pump laser for thermal mitigation, the maximum output powers can be further improved to 312 mW for 2717 nm laser and 145 mW for 2740 m laser at higher pump powers. Such a mid-infrared microchip laser source with very compact size could be have great potential in various eye-safety-related applications.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
During the past decades, mid-infrared lasers near 3 μm have attracted a lot of interest because of their important applications in remote-sensing technology, atmospheric-environment monitoring, and especially in biomedical science, due to their strong water absorption and the penetration depth of a few microns in biological tissue [1,2]. Er3+-doped materials make it possible to achieve 3-μm laser sources, which corresponds to laser transition from 4I11/2 to 4I13/2 by pumping ground-state (4I15/2) Er3+ ions to 4I11/2 level using compact InGaAs laser diodes emitting around 970 nm [3–5]. With the rising application need and with the development of solid-state laser technology, in the recent years, mid-infrared Er3+-doped lasers based on some novel host materials or ceramic materials have been intensively reported, such as Er:CaF2 ceramic [6], Er:SrF2 [7–9], Er:YAP [10], Er-doped CaxSr1-xF2 crystal [11], Er-, Pr-codoped GYSGG [12] and Er-doped sesquioxides [13,14].
Microchip lasers have attracted a lot of research interest because of many desirable advantages of them. The small sizes make them become welcome laser sources for various applications. Moreover, in general, microchip lasers have good beam quality and narrow spectrum linewidth, which makes them desirable for some particular utilization [15–17]. However, laser sources in the form of microchip with thickness less than 1 mm have also suffered an unavoidable problem: relatively low output power arising from weak absorption efficiency. This problem can be significantly reduced for Er3+-doped lasers operating at mid-infrared wavelength range of about 2.7 μm. In order to alleviate self-termination effect, concentration-dependent upconversion process for Er3+-doped laser should be taken into account, which requires a relatively high dopant concentration. As a result, it offsets the low absorption and allows moderate output power.
At present, it has not yet been attached enough importance to mid-infrared microchip laser that combines the application value of mid-infrared lasers and merits of microchip lasers. Reports on this research topic are indeed very rare. In 2015, You et al. reported a single-longitudinal-mode laser at 2.7 μm with an output power of 50.8 mW using a 0.6-mm Er:GGG microchip [18]. However, the separated input mirror (not direct coating onto the front surface of the microchip) destroyed the compactness of the laser system. Recently, we have made efforts on this research by configuring a sandwich structured laser resonator with Er-doped crystal length of 5 mm [19]. However, this is not a real microchip laser and it does not share the full advantages of microchip lasers. In 2012, a commercial Er:YAG microchip laser at 2940 nm with output power of 1.5 W was used as pump source of a Fe:ZnSe laser [20]. However, no any detailed report about the Er:YAG microchip laser can be found in [20].
In this contribution, an Er3+-doped sesquioxides Y2O3 ceramic with a thickness of 800 μm was used as laser gain medium, which shows great promise due to its superior thermal conductivity and one of the lowest maximum phonon energies among Er-doped gain materials. This is the first real MIR microchip laser with ceramic material as gain medium, to the best of our knowledge. Ceramic laser materials have attracted rising interest because growing large-size and highly doped ceramics is relatively easy compared to growing crystals. Moreover, growing ceramics is also less expensive. The Er:Y2O3 ceramic microchip lasers have been successfully operated not only at 2717 nm but also at 2740 nm. Thanks to the merit of microchip configuration, the achieved mid-infrared lasers have been checked to be single-mode oscillated.
2. Experimental setup
The laser experimental setup is schematically shown in Fig. 1. The pump source is a fiber-coupled 976-nm diode laser with core diameter of 105 μm and numerical aperture of 0.22. The maximum output power of the pump source is about 28 W. The pump beam was collimated by a doublet lens with focal length of 40 mm and then focused by another 50 mm doublet lens into the laser gain medium, which leads to a pump beam waist size of about 131 μm. The laser gain medium is a Er:Y2O3 ceramic with dopant concentration of 15at.%. In general, gain medium with thickness of less than 1 mm is called microchip [21,22]. In this experiment, the used Er:Y2O3 ceramic was cut and polished to a thickness of 800 μm. In order to mitigate thermal lensing effect originating from a large quantum defect (more than 64%), the Er:Y2O3 ceramic was attached to a water-cooled copper block with temperature set at 14 °C. A piece of indium foil was used between the Er:Y2O3 ceramic and copper block to strengthen thermal contact.
The input mirror coating was directly deposited onto the front surface of the Er:Y2O3 ceramic with a high transmission of more than 94% at pump wavelength and a high reflection of more than 99.8% at considered wavelengths. According to [23], the emission intensity of 2740 nm line is weaker than the 2717 nm line. Hence, several flat mirrors were subsequently used for output couplers (OCs) in order to exploring the laser performances at different emission lines. OC1 and OC2 were used for operating 2717 nm laser and they have transmissions of 1.0% and 3.6%, respectively. OC3 has transmission of more than 15% at 2717 nm and 1.3% at 2740 nm. It leads to suppression of the high-gain 2717 nm line and therefore the relatively low-gain 2740 nm emission line oscillates first.
3. Results and discussion
Single-pass absorption of pump power for the Er:Y2O3 ceramic was measured to be about 29.9% under no lasing condition. We deduced an effective absorption coefficient of about 4.44 cm−1, which is about 1.53 times higher than that of a 7at.% doped Er:Y2O3 ceramic reported in our previous work [24]. Nominal dopant concentration of the present Er:Y2O3 ceramic is 15at.%, i.e. about 2.14 times higher than the previously used Er:Y2O3 ceramic. The discrepancy between the two cases should be probably ascribed to the pumping wavelength linewidth. The 2717 nm laser was firstly reported by using OC1 and OC2, as shown the output powers in Fig. 2. Using OC1 with a lower transmission, the threshold power was about 350 mW and the maximum output power reached 147.7 mW at an absorbed power of 2.99 W, which leads to a slope efficiency of about 6.7%. Using OC2, laser threshold increased to about 540 mW, while the maximum output power promoted to 234.8 mW. The slope efficiency is about 10.9% by linear fitting the data. The slope efficiency is very comparable to our previous report (11.8%).
Fig. 2 Output power versus absorbed power of 2717 nm lasers using OC1 (transmissivity: 1.0%@2717 nm) and OC2 (transmissivity:3.6%@2717 nm); inset: typical laser spectrum.
Compared with [18] reporting a far less output power (50.8 mW) and power saturation at absorbed power less than 1 W with Er:GGG microchip, our present results indicated that Er:Y2O3 ceramic laser is a very promising candidate in microchip scheme for potential applications. Using the Findlay-Clay analysis, we estimated the resonator round-trip loss at the lasing wavelength. For the two OCs, Findlay–Clay formula can be transformed as Pth,1/Pth,2 = (L-lnR1)/ (L-lnR2), in which the laser threshold (Pth) is measured as a function of output coupler reflectivity (R), we then extrapolated an intracavity round-trip loss L of approximately 3.9%.
The typical laser wavelength is reported as an inset in Fig. 2 with a peak at 2717.1 nm. The FWHM was measured to be about 0.16 nm. The present laser resonator has a wavelength spacing of about 2.4 nm (97.6 GHz). Therefore, we can safely conclude that the laser operating in single longitudinal mode. The maximum output power is reported as 234.8 mW. Output power fluctuation was also observed. We recorded the output power stability in 20 minutes and the result is shown in Fig. 3(a) with power instability of about 3.9% withrespect to average output power during the 20 minutes, while 5.5% for the 234.8 mW maximum output power. Beam quality of the output laser at maximum output power was also evaluated by measuring the beam spot size at different distance after a focusing lens, as shown in Fig. 3(b). Using these data, beam propagation factor M2 can be estimated to be about 1.23 and 1.45 in x and y directions, i.e. close to diffraction limit. The beam spot is also shown as an inset in Fig. 3(b), which clearly displays a Gaussian fundamental mode with good circular symmetry. The good beam quality benefits from the microchip configuration, which could be very crucial for some applications.
Fig. 3 (a) Output power stability in 20 minutes and (b) output beam spot sizes in x and y directions at different distances achieved with OC2; inset: beam spot.
According to [23], emission spectrum of Er:Y2O3 ceramic has obvious multi-peak structure. The 2740 nm emission line is one of the four dominated intense peaks with emission cross section of about 0.75 × 10−19 cm2, which is about 85% that of 2717 nm line. As a result, the 2740 nm emission is suppressed in general. To realize lasing at this specific line, special coating is need, as we used the OC3 in this experiment. Figure 4 shows the results in regard to a 2740 nm microchip laser with OC3. The laser threshold is at about 0.64 W and the achieved maximum output power is about 102 mW, which leads to a slope efficiency of about 4.9%. The output power stability was also measured to be about 3.1% with respect to average output power and 4.5% to maximum output power in 20 minutes. The laser wavelength, shown as an inset in Fig. 4, peaks at 2740.9 nm with FWHM of about 0.15 nm. Beam quality of the 2740 nm laser was measured with M2 factors of about 1.15 and 1.26 in x and y directions.
Fig. 4 Output power versus absorbed power of 2740 nm lasers using OC3 (transmissivity: 15% @2717 nm and 1.3%@2740 nm); inset: laser spectrum and beam spot.
Under the two cases, the maximum output powers have both been limited by obvious thermally induced power saturations, which can be found in Fig. 2 and Fig. 4. Before about 2.5 W, the output powers keep linear increases. However, further increasing the absorbed power led to rollover of the output power curves. Previously, we found that the power saturation was not found until an absorbed power up to about 8 W for a relatively weakly doped Er:Y2O3 ceramic (7at.%) [24]. In [25], using 7at.% doped bulk Er:Y2O3 ceramic and by expanding pump beam to larger size, Wang et al. achieved a maximum output power of 2.05 W thanks to a mitigated thermal saturation effect. However, with a 15at.% doped sample, output power saturation occurred at an absorbed power of about 6 W [25]. The early saturation effect in this work should ascribe to the relatively high dopant concentration, on the one hand. On the other hand, compared to bulk material, cooling may be not so efficient for microchip since only one surface of the Er:Y2O3 ceramic keeps contact with the cooler. Additionally, considering our smaller pump beam size (132 μm versus 420 μm [25]), it seems reasonable for the early thermal saturation in the present work.
Further, we theoretically analyze thermal effect of the Er:Y2O3 ceramic by estimating its thermal focal length, which can be expressed as
where K is the thermal conductivity, ωp≈66 μm the average pump beam size in radius, η≈64.1% the fractional thermal loading, Pin the pump power, dn/dT≈8.5 × 10−6/K the thermal-optical coefficient [26], a≈4.44/cm the effective absorption coefficient and lc the laser crystal length. After simple calculation, one can find that the thermal focal length is about 12 mm at absorbed power of 3 W if taking the thermal conductivity 15 × 10−6 W/mK. A thermal conductivity of about 1 × 10−6 W/mK leads to thermal focal length of about 0.8 mm, which is just our present case. It should be pointed out that undoped Y2O3 ceramic has been reported to have high thermal conductivity, e.g. 17 W/mK [27], even better than YAG. However, as we know, with the increase of dopant concentration (15at.% doped for our Er:Y2O3), thermal conductivity could deteriorate significantly. Hence, here, the estimated value (1 × 10−6 W/mK) is probably reasonable. Precise measurement should be conducted in the future to verify the value of thermal conductivity.Last but not least, under the present experimental condition, we have considered power improvements for the two lasers by using a mechanical chopper with 50% duty cycle to modulate the pump power. Under a repetition rate of 100 Hz of the chopper, the output power saturation cannot be found until the absorbed power increased to about 4.8 W, i.e. an average absorbed power of 2.4 W. Under this situation, the maximum average output powers promoted to about 312 mW for 2717 nm laser and 145 mW for 2740 m laser.
4. Conclusions
To summarize, we have demonstrated compact mid-infrared microchip lasers at 2717 and 2740 nm using a 0.8-mm Er:Y2O3 ceramic. In continuous-wave regime, the maximum output power reached 234.8 mW at 2717 nm, while 102 mW for the 2740 nm laser. Benefiting from the compact configuration of the microchip, the two lasers have been found to have narrow line-widths and excellent beam qualities, which is very advantageous for many applications. Using a mechanical chopper, the maximum output powers have been improved to 312 mW for 2717 nm laser and 145 mW for 2740 m laser. Further work will focus on the optimizations for scaling the output power to watt-level, including reducing the dopant concentration to an optimum, optimizing the pump beam size and transmission of the OC, as well as enhancing thermal management.
Funding
National Natural Science Foundation of China (NSFC) (61575164, 61575088, 61861136007); Natural Science Foundation of Fujian Province of China (2018J01108).
Disclosures
The authors declare no conflicts of interest.
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