1. Introduction

Lasers operating at near-infrared (NIR) wavelengths have been receiving considerable attention as the application of optical devices is expanding into biomedical imaging and optical therapy [1, 2]. When the light exposed on the surface of animal tissue, NIR light with wavelengths of 800–1000 nm is most efficient for the deep penetration through the tissue, and therefore, in-depth information about the tissue under human skin can be obtained by performing NIR optical tomography [3]. Moreover, NIR is very effective in detecting biomolecules dissolved in a saline solution because of good transmission through water [4].

NIR lasers with wavelength-tuning capability could be widely utilized for multi-wavelength biomedical applications. In a diffuse optical tomography system, multiple light sources with different wavelengths could be replaced by a single tunable laser [3]. Spectral-domain optical coherence tomography requires lasers with widely tunable wavelengths to obtain 3D tissue images [5]. For detecting oxygen concentration carried by hemoglobin, absorption spectrum change depending on the ratio of oxy-hemoglobin could be measured by using a single tunable laser [6].

There are various approaches for realizing tunable lasers for NIR wavelengths. Thermal heating of vertical-cavity surface-emitting laser chips enabled wavelength tuning within a small range [7]. High speed tunable laser was demonstrated using a lightweight mirror with a high-contrast subwavelength grating [8]. A flexible membrane reflector was used to realize wavelength tuning by over 25 nm [9]. Fiber-optic tunable lasers developed for optical coherence tomography exhibited a considerably wider tuning range [10, 11]. A GaN-laser-diode-pumped tunable laser exhibited tuning in the range from visible to NIR wavelengths by incorporating a rotating broadband mirror and a dispersive prism [12].

In this work, the NIR tunable laser is demonstrated in terms of polymer waveguide technology. Studies on polymer waveguide devices have focused on devices operating at 1550 nm wavelength for optical communications [13, 14]. Development of polymer materials was also focused on reducing the absorption loss at that wavelength by incorporating fluorinated compounds into the polymers [15]. At a shorter wavelength, the absorption loss decreases, while the scattering loss due to the waveguide roughness increases considerably. Several research works have focused on the development of polymer waveguide devices operating at around 800 nm [16, 17]. However, based on polymer waveguide technology, tunable lasers operating at this wavelength have never been realized. The tunable external-cavity laser is constructed with a polymeric Bragg reflector tunable by thermo-optic (TO) effect and a semiconductor optical amplifier to provide optical gain. Low-loss-polymer waveguides for NIR wavelengths are designed, third-order Bragg reflector is incorporated to facilitate grating fabrication, and a thin-film micro-heater is fabricated for efficient TO tuning of the reflection wavelength. In the external-cavity laser, wavelength tuning by 21 nm was achieved by applying 1 W on the thin-film heater. The laser exhibited single-mode operation with a large side-mode suppression ratio (SMSR) and a narrow-bandwidth lasing spectrum.

2. Design and fabrication of the polymer-waveguide Bragg reflectors

To design the waveguide structure suitable for the NIR wavelength range, low-loss fluorinated polymer materials obtained from ChemOptics, Co., were used, which have the refractive indices of 1.455 and 1.430 for 1550 nm. The refractive index increases to 1.462 and 1.435 for 830 nm. The large refractive index contrast between the two polymers is suitable for increasing the reflectivity of surface relief grating even for shallow etch depth. However, in spite of this large contrast, an oversized rib structure is required for producing a single-mode waveguide with a moderate core size [18]. The single mode waveguide is designed to have a thickness of 3.5 μm, width of 6.0 μm, and rib height of 2.0 μm. Consequently, the effective refractive index contrast between the core and the lateral cladding becomes 0.0037.

The external-cavity tunable laser, as shown in Fig. 1 , consists of a polymer-waveguide Bragg reflector with a thin-film heater and a semiconductor optical amplifier (SOA) with a high-reflection coating on one side. The Bragg grating is located under the waveguide core layer. Because the period of first order grating is too small to fabricate, third-order grating with a period of 863 nm is fabricated by using 442-nm laser interferometry. The effect of grating depth on the reflectivity is evaluated as shown in Fig. 2 . Effective index modulation by the grating pattern is determined by calculating the effective index, and then, the reflection and transmission spectra are obtained by using the transmission matrix method. For various grating lengths, the reflectivity is calculated as a function of grating depth (Fig. 2(b)). It is shown that third-order grating is good enough for realizing a reflectivity of over 50% with a grating length of 3 mm.

 

Fig. 1 Schematic diagram of external cavity NIR lasers consisting of a polymer waveguide with a TO tunable Bragg grating and a SOA with a high reflection coating on one end.

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Fig. 2 (a) Transmission and reflection spectra obtained using the transmission matrix method for the third-order grating with a length of 5 mm and an etch depth of 200 nm. (b) Bragg grating reflectivity as a function of the grating etch depth for various lengths and orders.

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The extent of wavelength tuning is proportional to the refractive index change of the polymer material. Given an initial wavelength of 800 nm, if one needs to realize a wavelength tuning of 8 nm, the refractive index needs to be changed by 1%; such a change is difficult to realize in materials other than polymers. The low-loss polymer LFR (product of ChemOptics Inc.) has a TO coefficient of −2.567 × 10⁻⁴/°C, and for a temperature change of 50 °C, its refractive index changes by 1%. The ZPU polymer used in this study has a TO coefficient of −1.8 × 10⁻⁴/°C, and a temperature change of 80 °C is required to achieve 1% tuning. In addition, due to the low thermal conductivity of the polymer, the device can be driven with less power. The thin-film heater is formed over the upper cladding layer, as shown in Fig. 1. The temperature change obtained by applying electrical power depends on the structure of the thin-film heater, the total thickness of the waveguide, and the thermal conductivity of the polymer material [19]. For realizing a temperature change of 80 °C at the waveguide center, a heating power of 324 mW is required when the total polymer thickness is 20 μm and the thermal conductivity of the ordinary polymer material is 0.17 W/m·K.

Tunable polymeric Bragg reflectors are fabricated by employing typical fabrication processes such as spin coating, photolithography, and dry etching, as outlined in Fig. 3 . On a Si wafer, the polymer ZPU13-430 with a refractive index of 1.435 (at 800 nm) is spin coated to form the lower cladding layer with a thickness of 16 μm. The polymer is cured for 3 min in a UV chamber and then hard baked at 160 °C for 30 min. For the fabrication of the Bragg grating, a 442-nm He-Cd laser is utilized to produce an interference pattern on a photoresist, and then, the pattern is transferred to the lower cladding layer by following oxygen-plasma reactive ion etching. In the fabricated device, the grating depth and length become 200 nm and 7 mm, respectively, which results in a reflectivity of 92% according to the design result.

 

Fig. 3 Schematic of the fabrication procedures for the polymeric waveguide with a Bragg grating. The device consists of rib-type waveguide, and the Bragg grating is placed under the core layer.

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ZPU13-455 polymer with a refractive index of 1.462 (at 800 nm) is spin coated over the surface relief grating pattern to form the core layer. Consequently, the core thickness varies from 3.5 μm to 3.7 μm in the grating. The waveguide pattern is defined on the core layer by conventional photolithography using AZ5214 photoresist. The ZPU polymer used for the lower cladding is spin coated once again to form an upper cladding layer. The thin-film heater is fabricated by thermal evaporation of Al and by employing a conventional patterning process. Additional glass blocks were attached to the waveguide, the waveguide device was polished, and then, the single-mode fiber was pigtailed.

3. Measurement of tunable laser characteristics

The spectral response of the fabricated tunable Bragg reflector device was characterized by using an SOA with a center wavelength of 838.8 nm and a 3-dB bandwidth of 55.7 nm (Superlume-Diodes Ltd., SOA371). A high-reflection coating was applied on one side of the SOA, and the other side was subjected to anti-reflective treatment so that the SOA can be used as a gain medium in the external-cavity laser. During the alignment, a streak of propagating light was observed through a visible-wavelength CCD, as shown in Fig. 4 . As soon as the alignment was optimized to overcome the cavity loss, the intensity of scattered light significantly increased, as shown in Fig. 4(b). The propagation loss of the straight waveguide at NIR wavelength was measured to be 1.2 dB/cm in the cutback experiment. Fiber-to-fiber insertion loss measured from a 1-cm long straight waveguide with a grating was about 3 dB. The propagation loss was increased for the NIR wavelength due to the side wall roughness.

 

Fig. 4 Images of light streaks observed by using a visible CCD during alignment of optical fiber on a polymer waveguide: (a) before optimized alignment and (b) after alignment is optimized to realize the lasing.

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The output spectrum of the external-cavity laser, which exhibits an SMSR of over 40 dB, is shown in Fig. 5 along with an inset of 0.05 nm resolution measurement result. The lasing spectrum exhibited 20-dB bandwidth of 0.2 nm and 3-dB bandwidth of 0.05 nm, which were limited by the resolution of spectrum analyzer. The polarization dependence of the device was examined as shown in Fig. 5(b). A fiber-optic polarization controller was inserted to adjust the excited polarization on the waveguide to be TE or TM. The laser exhibited an initial lasing wavelength of 849.2 nm for TE mode, and 848.6 nm for TM mode. The output power was about 0 dBm regardless of the polarization, which implies that the propagation loss of the polymer waveguide does not depend on the polarization. The lasing wavelength difference between the two polarizations was 0.6 nm corresponding to the birefringence in the polymer waveguide, 0.00137 (approximately 0.1%).

 

Fig. 5 Output spectrum of external cavity NIR laser: (a) the spectrum shows an output optical power of 1.2dBm, a 20-dB bandwidth of 0.2 nm, and a side-mode suppression ratio of 40 dB; and (b) the polarization dependence of the laser with an initial lasing wavelength of 849.2 nm for TE mode and 848.6 nm for TM mode

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The power efficiency of the tunable laser was measured and compared to that of the SOA, by measuring the L-I curve as shown in Fig. 6 . The output power of SOA increased gradually like the case of an LED, while the output of the external-cavity laser suddenly increased as the current rose beyond the threshold of 36 mA. The slope efficiency of the tunable laser was found to be 0.069 W/A, which was slightly lower than that of the 1550-nm external-cavity laser used in our previous demonstration [13]. To verify the TO effect of the ZPU polymer for NIR wavelength, the device was placed in a temperature-controlled oven to observe the change in the peak wavelength as a function of temperature. When the temperature was raised from 22 °C to 100 °C, the peak wavelength was changed by 11.36 nm, which was corresponding to TO coefficient of −2.5 × 10⁻⁴/°C at 850 nm.

 

Fig. 6 Lasing behavior of a polymer-waveguide tunable laser represented by the L-I curve with a slope efficiency of 0.069 W/A.

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In the wavelength-tuning experiment, electrical heating power was applied on the thin-film heater with a stepwise increase by 50 mW. As shown in Fig. 7(a) , the peak wavelength of lasing spectrum shifted toward shorter wavelengths as the temperature increases. The output wavelengths were tunable by 21 nm for the maximum applied power of 1 W. Figure 7(b) shows the linear response of the peak wavelength to the applied electrical power with a tuning efficiency of −20.82 nm/W. The efficiency of the 1550-nm laser was −170 nm/W. It should be noted that the LFR material used for 1550 nm laser had considerably higher TO coefficient than the ZPU material used in this work. Moreover, for a given TO index change, the larger extent of tuning is possible in the case of longer wavelength. In the waveguide structure, the vertical position of the waveguide core also affects the extent of TO tuning efficiency. For improving the power efficiency of the NIR device, optimization of the device structure is required to have increased polymer cladding thickness with the waveguide core located close to the heater. Incorporation of the LFR material along with the optimized structure could reduce the operating power significantly.

 

Fig. 7 Wavelength-tuning characteristics of the polymer-Bragg-grating tunable laser: (a) wide tuning from 848.7 to 827.9 nm by applying electrical power increasing with steps of 50 mW,(b) peak wavelength position as a function of applied electrical power, and (c) continuous tuning for a narrow tuning range with a step-wise change in wavelength by 0.1 nm in each step.

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During the wavelength-tuning experiment, the output-power fluctuation was not significant within the tuning range of 10 nm. However, the power decreased to about 10 dB in the case of the maximum tuning of 21 nm, which was caused by the radiation of the guided mode as the index decreases. The power decrease in the large tuning could be prevented by increasing the refractive index contrast of the waveguide. In particular, the contrast between the core and upper cladding needs to be increased by considering the significant temperature gradient produced by the micro-heater. The slight fluctuation in the output power at smaller tuning ranges might be caused by the unstable alignment of the fiber and the change in polarization. To demonstrate that the device was suitable for precise and continuous wavelength tuning with no wavelength hopping, as shown in Fig. 7(c), the device was tuned to realize stepwise wavelength change by 0.1 nm through a precise control of the electrical heating power.

4. Conclusion

In conclusion, a polymer waveguide tunable laser operating at NIR wavelengths is demonstrated with a performance useful for practical applications. The laser exhibited an output power of 0 dBm, an SMSR of 40 dB, and a tuning range of over 20 nm. The peak-wavelength change was linearly proportional to the applied heating power, and the wavelength was precisely tunable in steps of 0.1 nm without wavelength hopping. The NIR tunable laser could become an important building block in extending potential applications of polymeric integrated optics, especially in the field of biophotonics.