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Abstract
We present a broadband wavelength-swept laser using a 16-µm-core-diameter Cr4+:YAG crystal fiber as the gain medium. The laser-diode-pumped crystal fiber laser has a threshold of only 102 mW due to the low propagation loss and high heat dissipation efficiency. The laser achieves a sweeping wavelength range of 134 nm, centered around 1425 nm, with a scanning speed of 163 k nm/s. Notably, the cross-polarization-coupled excited state absorption of the signal wavelength constrained the long-wavelength lasing limit. This laser has the potential for swept source optical coherence tomography applications, providing an axial resolution of 11.4 µm.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Wavelength-swept lasers can be considered high-speed tunable wavelength lasers offering broadband emission with instantaneous narrow linewidth [1–3]. In biomedicine, swept-source optical coherence tomography (SS-OCT) has experienced significant development over the past two decades [4]. The work aims to develop a broadband near-infrared (NIR) swept laser for high-spatial-resolution OCT applications. NIR wavelengths benefit deep tissue penetration in biomedical applications. To achieve cellular-resolution OCT, the ultra-broadband emissions from transition-metal-ions-doped gain media have advantages compared to semiconductor optical amplifier based tunable lasers. Various transition metal ions have shown broad NIR tunable lasers [5–8]. Specifically, Cr4+:YAG has demonstrated superior performance in the 1.4-µm wavelength range. The development of Cr4+:YAG lasers with tunable wavelengths spans over 30 years. In 1988, N. B. Angert et al. created a gain-switched laser with a tunable range of 1.35–1.45 µm [9]. Subsequent research indicated that using gratings with improved dispersion could achieve a narrower instantaneous linewidth [10]. High-power laser diodes (LDs) have been used as the pump light source to reduce system complexity and cost, but a pump power of 2 W is still required [11]. Even with a high-reflectance output coupling mirror (T < 0.3%), the lasing threshold remains approximately 1 W [12]. Furthermore, the low efficiency of the system requires a water cooling system to mitigate the thermal effects encountered by the gain block [13], resulting in a substantial increase in the volume and complexity. On the other hand, drawing the bulk material into a crystal fiber can significantly improve the surface-to-volume ratio, effectively reducing thermal effects. By employing a crystal fiber with a core diameter of 120 µm, the tuning range of a Cr4+:YAG laser can reach 180 nm, but the lasing threshold remains at 2.1 W [14]. With an external-cavity configuration, the tunable wavelength range using birefringent filters can reach 156 nm (1353–1509 nm) using the Cr4+:YAG crystal fiber as the gain medium [15].
Currently, commercial tunable wavelength lasers operating in this specific band face limitations imposed by the bandwidth of a semiconductor optical amplifier. As a result, their tunable range is restricted to approximately 130 nm, and their output power is limited to only 10 mW. Another existing approach involves using a 65-meter-long bismuth-doped fiber, which offers a broader tuning bandwidth spanning from 1366 to 1507 nm. However, this system is based on a two-stage Raman laser for the pump, which leads to increased complexity, and the width of the laser line is constrained to only 0.15 nm [16]. Consequently, developing tunable wavelength lasers with broadband capabilities, a low lasing threshold, and a narrow linewidth remains challenging.
In this work, we demonstrate a wavelength-swept Cr4+:YAG laser for the first time. It covers a range of 134 nm, is centered at 1425 nm, and operates at a scanning speed of 163 k nm/s. Laser performance is achieved using crystal fiber-based gain media fabricated through the laser-heated pedestal growth method [17,18]. The co-drawing laser-heated pedestal growth method fabricated the double-clad Cr4+:YAG crystal fibers. During the drawing process, interdiffusion exists between the silica capillary and the Cr4+:YAG core. So, the inner clad is a mixture of silica and YAG, and the crystalline YAG core is reduced. The core diameter depends on the CO2 laser power and the drawing speed [19]. This method produces cylindrical-shaped fibers that enhance heat dissipation and significantly reduce the lasing threshold thanks to their micron-level core size [20]. We have used selected area electron diffraction and electron backscattered diffraction to verify the crystallinity of the core. The NIR wavelength range is advantageous for deep probing biological tissues due to low tissue scattering and absorption [21–23]. Additionally, the long coherence length of the swept laser enables extended axial range imaging, which is crucial for applications such as endoscopy and imaging of the anterior segment of the human eye [24].
2. Wavelength-swept Cr4+:YAG crystal fiber laser
2.1 Numerical model
The [Ar]3d2 electronic configuration of the Cr4+ ion has only two free electrons in the 3d electron orbital region. They are susceptible to the influence of the crystal field. For an ideal tetrahedron (Td symmetry) site, there are three S4 symmetry axes. However, when Cr4+ is placed in the YAG matrix, the distorted tetrahedron will be elongated along the crystal axis and reduced to the D2d symmetry with only one S4 symmetry axis [25]. Therefore, the Cr4+ ions in the YAG crystal can be divided into three groups, as depicted in Fig. 1(a). The excitation directions are divided into the π direction, parallel to the S4 axis, and the σ direction, perpendicular to the S4 axis. The absorption cross-sectional areas for the 1064-nm pump light are ${\sigma _\pi } = 3.9{-}5.0 \times {10^{ - 18}}\; \textrm{c}{\textrm{m}^2}\; \textrm{and}\; {\sigma _\sigma } = 1.5{-}1.9 \times {10^{ - 19}}\; \textrm{c}{\textrm{m}^2}$ [26]. The disparity between these values exceeds 26 times, making π polarization the preferred method for effectively exciting the Cr4+ ions.
Fig. 1. (a) The YAG structure scheme with three orientation types of the Cr4+ centers. The π and σ polarizations are respectively parallel and perpendicular to the S4 local symmetry axis. The photo insert shows an image of a double-clad Cr4+:YAG crystal fiber. (b) Simplified Cr4+:YAG energy diagram. Solid arrows: radiative transitions. Dotted arrows: non-radiative transitions.
To predict the performance of Cr4+:YAG crystal fiber lasers, and optimize their physical parameters, we have adopted a simulation code based on the original work of Jheng et al. [27]. In general, Cr4+:YAG can be considered as a 4-level laser material with parasite pump and signal excited-state absorptions (ESAs). The simplified energy diagram is shown in Fig. 1(b). The energy levels that can interact with the pump or fluorescence are numbered from levels 0 to 5. The rate equation describing the dynamics of the excited state population can be written as follows:
The pump light propagated in the Cr4+:YAG crystal fiber encounters ground state absorption, excited state absorption, and propagation loss. We adopt a distributed model based on the relaxation method to accurately predict the output performance of Cr4+:YAG crystal fiber laser sources [15,28]. The equations describing the evolution of the pump power (Pp) and signal power (Ps) in the crystal fiber at the axial position z are:
Table 1. Summary of optical parameters of the Cr4+:YAG crystal fiber
2.2 Cr4+:YAG crystal fiber laser
The experimental setup for the LD-pumped Cr4+:YAG crystal fiber laser is shown in Fig. 2. The gain fiber has a core diameter of 16 µm and a length of 4.7 cm. The Cr ion doping concentration is about 0.5-mol.%. The crystal fiber was free-space pumped by a 1064-nm fiber-pigtailed LD (LD-1064-BF-600, Innolume) with a linearly polarized output. Due to the high numerical aperture of the Cr4+:YAG crystal fiber, the pigtailed LD’s coupling efficiency is typically more than 85%. An optical isolator (IO-3D-1064VLP, Thorlabs) was inserted to prevent backward reflection. In addition, a half-wave plate was positioned before the crystal fiber to control the relative orientation between the pump light polarization and the crystal axis. With the use of the test module, the laser cavity was formed by a multilayer coating deposited on the pump end and a flat output coupler. The multilayer coating provides high reflection (HR, R = 99.6%) for the laser signal and an anti-reflection (AR, R = 8.4%) for the pump light. The output coupler was designed to have a 5% transmittance at 1350–1550 nm and HR at the pump wavelength to recycle the residual pump light. The output end of the crystal fiber was AR coated to minimize the signal loss. An aspheric lens was used to collimate the light coming out of the crystal fiber. The crystal fiber was mounted on a copper heat sink and covered with silver gel to help heat dissipation. The primary heat source is the quantum defect (∼25.3% of the pump’s photon energy). The ESA may cause some additional heat. A dichroic mirror (LPD01-1064RS-25, Semrock) was employed to monitor the backward laser powers and spectra, and a long-wavelength pass filter was used to block the residual pump. To enable the wavelength sweeping, we incorporated the tuning module, consisting of a holographic grating (53004BK02-246 H, Richardson Gratings) mounted in Littrow configuration and a galvo mirror, into the laser cavity. The signal polarization matched the TM polarization of the grating. The grating pitch was 1050 grooves/mm, with high wavelength selectivity, for generating narrow laser linewidth.
Fig. 2. Schematic of the grating-based Cr4+:YAG crystal fiber laser with test and tuning modules. PMF: polarization-maintaining optical fiber, ISO: optical isolator, DM: dichroic mirror, HWP: half-wave plate, L1, L2: aspherical lens, DCCF: double-clad crystal fiber, grating: reflective holographic grating, LPF: long-wavelength pass filter. PM: power meter.
Figure 3 shows the measured laser output powers and polarization characteristics. When using a single LD pump, the output power can reach 38 mW at a pump power of 400 mW. The lasing threshold and slope efficiency were measured to be 51 mW and 10.7%, respectively. These laser output powers agree well with our simulation results. The fiber-based waveguide geometry features a high pump intensity, resulting in a low lasing threshold. The round-trip cavity loss is measured to be 0.58 dB. Figure 3(b) displays the laser output polarization. The diffraction grating was mounted with its groove in the vertical direction so that the laser beam was TM-polarized. The polarizer used in the experiment has an extinction ratio of 105. The measured polarization extinction ratio is 54.4, which is suitable for diffraction-grating-based wavelength tuning.
Fig. 3. (a) Experiment and simulation of the forward output power of the laser. (b) The measured polarization at a pump power of 560 mW.
2.3 Impact of pump LD polarization
This section focuses on analyzing the impact of the pump LD polarization, considering the presence of three groups of excitable chromium ions (Cr4+) oriented along mutually perpendicular crystal axes, as illustrated in Fig. 1(a). The polarization dependence of the ESA on Cr4+:YAG’s gain has been discussed in the literature [5]. It is speculated that ESA in both polarization directions affects the same ion population. The pump power was fixed at 563 mW, and a half-wave plate was positioned in front of the coupling lens (L1) to adjust the pump light’s polarization direction. Both the horizontally and vertically polarized laser output powers were then measured.
As the three S4 axes of Cr4+:YAG are symmetric with each other, they possess similar properties in principle. The pump laser propagated along the [29] direction of the Cr4+:YAG crystal fiber. Figure 4(a) shows the dependence of pump polarization direction on the laser output powers and polarization directions. When the pump polarization starts to rotate from 0° to 90°, the Cr4+:YAG laser powers in the horizontal and vertical polarization directions were measured at the same time. The laser output polarization under different pump polarization directions is illustrated in Fig. 4(b). Since the pump light in any polarization can be divided into [001] and [010] polarization components, the two S4 groups of ions can be independently excited. The results indicate that dual-polarization lasers were generated from the Cr4+:YAG crystal when the pump polarization was approximately 45°. As shown in Fig. 4(c), the slope efficiency and laser threshold under the pump of different polarization directions can be obtained to analyze the source of losses.
Fig. 4. (a) Dependence of the laser output power and polarization on the pump LD polarization at a pump power of 563 mW. (b) The laser output power as a function of pump polarization (c) Impact of the pump LD polarization on the laser output power under different pump powers.
In Fig. 4, it is evident that the total output power is lower when the pump polarizations are set between 30° and 60°, compared to when there is only one output polarization. This observation suggests that the Cr4+:YAG laser exhibits polarization-dependent loss. To analyze the source of these losses, we conducted measurements of the lasing threshold under different pump polarization directions, as depicted in Fig. 5. The threshold increase around the pump LD polarization of 45° will be discussed in detail in Sec. 3.1.
2.4 Wavelength tuning and sweeping characteristics
By adopting the above-mentioned tuning module, we are able to build a wavelength-sweeping laser. Figure 6(a) demonstrates the experimental results, revealing an instantaneous laser linewidth of 0.41 nm with a side-mode suppression ratio of more than 50 dB. Additionally, we determined the polarization extinction ratio to be 23.95, as depicted in Fig. 6(b). It requires an adjustment of Littrow mounting with approximately 8.06° to tune the laser wavelength from 1350 nm to 1525 nm. The grating diffraction efficiency is 97% within the 1300–1600 nm range. As shown in Fig. 6(c), this tunable laser spans a wavelength range from 1358 nm to 1491 nm. Overall, our laser system exhibited a low round-trip loss of 0.89 dB, leading to a remarkably low threshold of only 85 mW.
Fig. 6. The measured laser (a) instantaneous spectrum, (b) output polarization, and (c) tunable spectra from 1358 nm to 1491 nm. (d) Influence of wavelength sweep speed on the swept-laser wavelength range.
To enable the wavelength tuning, a grating-based galvanometer (GVS001, Thorlabs) tuning module was introduced into the laser cavity, as illustrated in Fig. 2. The surface of the galvanometer was gold-coated to enhance the reflectance of the signal light band. With the laser threshold of 115 mW, the pump power was increased to 232 mW for wavelength-swept testing. Limited by the maximum repetition rate, we increased the scanning angle of the galvanometer and reduced the duty cycle to obtain the laser characteristics at a high scanning speed. The wavelength-sweeping laser’s time interval is reduced, equivalent to a higher scanning speed. As shown in Fig. 6(d), when the maximum scanning angle increases, the corresponding scanning speed can reach 342 k nm/s, equivalent to the scanning repetition rate of 3.47 kHz, and the sweep wavelength range only drops from 103 nm to 98 nm. The 3-dB bandwidth is about 79.2 nm, equivalent to an axial resolution of 11.4 µm for OCT applications [2].
The pump power was increased beyond the signal saturation region of the Cr4+:YAG crystal fiber, as illustrated in Fig. 7, to explore the capabilities of wavelength tuning and sweeping. By manually adjusting the galvanometer’s driving voltage at a slow pace, the tuning range was expanded to 160 nm. Even when the galvanometer was set to a sweeping speed of 163 k nm/s, the wavelength-sweeping range remained at 134 nm. Figure 7(a) demonstrates that the tuning and sweeping wavelength range can be maintained when the Cr4+:YAG crystal fiber is operated at the signal saturation regime. Figure 7(b) further confirms that the sweeping range can be maintained by utilizing a sine-wave driving signal on the galvanometer with a repetition rate ranging from 50 to 500 Hz. This represents the first demonstration of a wavelength-sweeping laser utilizing Cr4+:YAG crystal fiber with a maximum sweep speed of 342 k nm/s. The galvanometer’s capabilities currently limit the maximum sweep speed.
Fig. 7. (a) The measured tuning and sweeping ranges of the Cr4+:YAG crystal fiber. The signal gain was estimated based on our simulations. (b) The wavelength range at different sweeping speeds with applied sine-wave galvanometer driving signal.
3. Discussion
3.1 Polarization-dependent pump threshold
To explain the pump polarization-dependent threshold, a simulated result is shown in Fig. 8. Only one of the S4 ion groups was considered, so the perpendicular crystal axis should be added when closing to the pump polarization of 45°. The simulation and experiment results agree reasonably from 0° to 30°. A more accurate simulation could be achieved if the rate equations of the pump and signal powers could be converted to vector equations. However, beyond 45°, the output power experiences a notable decrease attributed to the influence of the laser light, which is generated by the perpendicular crystal axis.
Fig. 8. Comparison between the measured laser output powers (solid lines) and the simulation results (dashed lines) of one excited S4 group of ions.
When the pump LD is polarized at 45°, the Cr4+ ions on the [001] and [010] crystal axes generate lasers in the direction of their respective crystal axes. As a result, cross-polarized laser outputs in both π and σ polarizations are produced. The cross-polarized emission can affect the signal light’s ESA, thereby raising the threshold. By analyzing the measured pump threshold power [9], it is possible to roughly estimate the emission cross-section and determine the ESA cross-sectional area in the σ direction. This can be achieved using the model proposed by Payne et al. [30], as depicted in Eq. (3).
where ${P_{th}}$ is the laser threshold, ${\sigma _e}$ and ${\sigma _{ESA}}$ are emission and ESA cross-sections, respectively. When the pump polarization direction is at 0 degrees, only the ESA in the π direction needs to be considered because there is no signal light in the σ direction. However, when the pump polarization is at 45°, both the π and σ directions must be considered. According to Eq. (3), the ratio of the signal ${\sigma _{ESA}}$ to ${\sigma _e}$ (${\sigma _{ESA,\; ({\pi + \sigma } )}}/{\sigma _e}$) is found to be 0.81. After deducting the contribution from the π polarization (${\sigma _{ESA,\; \; \pi }}/{\sigma _e}$) of 0.47, the value of ${\sigma _{ESA,\; \sigma }}/{\sigma _e}$ can be determined as 0.34. These ratios are comparable to those reported in the literature [5], where ${\sigma _{ESA,\; \; \pi }}/{\sigma _e}$ is reported as 0.40 ± 0.14, and ${\sigma _{ESA,\; ({\pi + \sigma } )}}/{\sigma _e}$ is reported as 0.73 ± 0.25. As depicted in Fig. 9, it is evident that the laser threshold increases when the σ polarization begins to lase, which proves that the ESA has polarization dependence.Fig. 9. Experiment and simulation of the output laser threshold when the pump polarization direction is 0°–45°.
3.2 Impact of excited-state absorption on the wavelength tuning range
As the pump power increases, certain wavelengths in the laser output may experience a decrease in power or even fail to sustain lasing. This indicates an additional loss for these wavelengths at high pump power. A similar observation was found in a previous study [30], in which wavelengths beyond 1450 nm were theorized to exhibit further loss and asymmetry in the tunable spectral shape due to ESA. In the relatively short-wavelength regime, i.e., 1350–1447 nm, the laser threshold increases as the lasing wavelength deviates from the central gain wavelength until the gain no longer overcomes the loss. In addition, experimental results demonstrate an asymmetrical tunable spectral shape at both the short-and long-wavelength ends. Specifically, the laser threshold in the long-wavelength range (>1460 nm) increases more rapidly compared to the short-wavelength range. Above 1480 nm, the laser threshold fluctuates.
The laser behavior above 1480 nm was measured to understand the relationship between the signal intensity and the ESA. When the pump power increases, the efficiency of these wavelengths gradually decreases, as shown in Fig. 10. The output power gradually decreases after a slow rise and even fails to lase at a high pump. However, the slope efficiency is linear at the short-wavelength range, even under maximum pump power. This excludes the thermal effect at high pump powers. Therefore, it is most likely attributed to the ESA caused by the increased signal light intensity in the DC Cr4+:YAG crystal fiber. Because of the small 16-µm core, the signal light intensity is high, resulting in greater ESA impact on the decreased slope efficiency.
4. Conclusion
In summary, the Cr4+:YAG crystal fiber demonstrates significant potential as a broadband gain medium for swept lasers operating in the near-infrared wavelength range, particularly for biomedical imaging applications. This unique fiber offers the ability to achieve deep tissue penetration with spatial resolution at the micrometer level. By manually tuning the galvanometer driving voltage at a slow speed, an impressive wavelength tuning range of 160 nm can be achieved. Even at a sweep speed of 163 k nm/s for the galvanometer operation, the sweeping range remains substantial, reaching 134 nm. The maximum achievable sweep speed is 342 k nm/s, limited only by the capabilities of the galvanometer employed. We have also observed and analyzed the impact of signal ESA on both the threshold pump power and the slope efficiency. Through thorough calculations of the laser threshold, we deduced that the ratio between the ESA cross-sectional area in the σ direction and the emission cross-sectional area in the π direction is 0.34. Given these remarkable properties, the wavelength-swept Cr4+:YAG crystal fiber laser holds great promise for utilization in SS-OCT applications, particularly in intravascular imaging scenarios.
Funding
National Science and Technology Council (111-2221-E-002-078-MY2, 111-2221-E-027-039, 111-2634-F-002-021).
Acknowledgments
The authors would like to thank the technical support from the National Center for High-Performance Computing.
Disclosures
The authors declare no conflicts of interest related to this article.
Data availability
Data underlying the results presented in this paper are not publicly available but may be obtained from the authors upon reasonable request.
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