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Abstract
Phase-shifted Bragg gratings have been extensively implemented in superior in-fiber bandpass filters or wavelength selectors, although high-temperature operation remains a challenge. We propose a phase-shifted type-IIa fiber Bragg grating (PSBG-IIa), which can conduct a notch signal as narrow as 4.8 pm within the stopband. The notch’s spectrum and wavelength can be adjusted according to the flexible design of the phase-mask translation. Using the thermal resistance as well as the narrow band notch, the PSBG-IIa is implemented in a distributed Bragg reflector laser structure to demonstrate a single longitudinal mode and single polarization laser output that can stabilize robustly at 500 °C. The results demonstrate that the proposed device qualifies as a high-quality optical regulator, without compromise, in the high-temperature region.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Phase-shifted fiber Bragg gratings (PSBGs) serve as outstanding in-fiber bandpass filters and wavelength selectors, and have attracted extensive attention in the fields of optical communications, microwave photonics, photonics sensors, and fiber lasers. Applications of PSBG include high-fineness filters in optical interconnections [1–3], high-resolution sensors for detecting extremely tiny changes induced by strain [4,5], transverse loadings [6], ultrasonic waves [7], refractive indexes [8], and bio-analytes [9]. Moreover, when inserted into a fiber laser cavity as a highly spectral selector, PSBG provides laser output with a single longitudinal mode and single polarization, enabling flexible design of laser cavities [10,11].
In recent years, an increasing need has emerged to implement fiber-based devices in extreme scenarios, such as high-temperature environments. For example, owing to their compact size, multiplexing capability, and high signal-to-noise ratio (SNR), distributed Bragg reflector (DBR) fiber lasers are effective for use as stable light sources or sensing elements in high-temperature environments. To achieve this aim, DBR fiber lasers with high thermal stability have been developed, using high-temperature-resistant fiber Bragg gratings as reflectors, including type-II gratings [12], type-I gratings [13,14], regenerated gratings [15], and type-IIa gratings [16]. Hence, exploiting high-temperature-proof fiber filters is essential for improving the performance of DBR lasers operating at high temperatures; however, studies have rarely been conducted in this important area.
In this paper, a phase-shifted type-IIa fiber Bragg grating (PSBG-IIa) is proposed to provide a high-performance bandpass filter that can operate at high temperatures. The PSBG-IIa is fabricated by administrating a modulation phase change between two successively inscribed type-IIa Bragg gratings. This fabrication technology offers the advantages of a simple structure and ease of implementation, in which the phase-shift amount is flexibly controlled and the grating spectrum can be tailored. The temperature endurance, strain response, and polarization of the PSBG-IIa are investigated. Taking advantage of the high-temperature-resistance as well as narrow bandpass width with a relatively low loss [17], the PSBG-IIa is applied in the cavity of a DBR fiber laser to achieve a single longitudinal mode and single polarization laser output that can operate stably in high-temperature environments. The results demonstrate that the proposed device qualifies as a high-quality optical regulator without compromise in the high-temperature region, enabling potential applications in the fields of optical communications, sensors, and lasers that require operation in extreme circumstances.
2. Fabrication of PSBG-IIa
For fabrication of the PSBG-IIa, a 193 nm excimer laser (BraggStar Industrial, Coherent) was used as the inscription source. Incorporating a rectangular aperture slot, the laser beam was transversally shaped into a rectangle with a size of 6 mm × 2.5 mm (height × width). The laser repetition was maintained at 200 Hz, while the energy density was adjusted to 120 mJ/cm2 per pulse by means of a cylindrical lens with a focal length of 10 cm that focused the laser beam to 200 μm in the height dimension. A phase mask (PM) with a pitch of Λpm = 1067.17 nm was used to transform the uniform laser beam into a periodic intensity distribution. A broadband light source with a wavelength range of 1250 to 1650 nm and an average power density of −30 dBm/nm was used as the referencing background. Two types of optical spectrum analyzers (OSAs), with resolutions of 0.02 nm (6370C, Yokogawa) and 0.16 pm (BOSA-lite, Argon Photonics), were employed to monitor the grating optical spectrum. The inscription beam and PM were driven parallel to the fiber axis. The fiber selected was a photosensitive Er-doped fiber (Fibercore) with a thin core diameter of 3 μm and conventional cladding diameter of 125 μm. The high numerical aperture (NA) of 0.24 but lower pump absorption of 5 dB/m indicated that the fiber was heavily doped with germanium, guaranteeing the prerequisites for the formation of the type-IIa grating with a negative index change [18–20].
Figure 1 illustrates the two-step method of the PSBG-IIa inscription. Firstly, as indicated in Fig. 1(a), step 1 was similar to the process discussed in our previous report [16]. In this case, the point-exposure method was adopted to ensure continuous exposure to the grating region, enabling generation of the type-IIa grating, which exhibits roll-over evolution [17]. By means of point exposure for 6 min, a negative-index Bragg grating was obtained, which could reflect 99% (−20 dB) of the transmission power at 1547 nm, as indicated in Fig. 1(b). Secondly, as illustrated in Fig. 1(c), the PM was precisely displaced by Λ/2 through a high-precision linear translational stage (P611.1s, PI), where Λ = Λpm/2, which denotes the Bragg grating pitch, to introduce a π-phase-shift into the refractive index modulation of the fiber core. Meanwhile, the laser was shifted by a distance across an entire beam width of 2.5 mm, aiming at a blank part of the fiber close to the originally exposed position. Thereafter, using the same irradiation method as in step 1, a PSBG-IIa with a strength of 35 dB was obtained. The total grating length was 5 mm. According to Fig. 1(d), a notch signal located at 1547.028 nm could be clearly observed in the middle of both the transmission and reflection spectra, denoting that a π-phase shift was successfully introduced into the modulation. To investigate the grating spectrum further, a BOSA with a 0.16 pm resolution was applied to interrogate the notch signal. As illustrated in Fig. 2(a), slight distortion and loss of the transmission spectrum were present, although the notch was clearly visible. From the inset of Fig. 2(a), it is clear that the notch exhibited a relatively narrow 3 dB bandwidth of 4.8 pm, signifying a Q-factor of 3.2 × 105. Furthermore, by using the two-step inscription method, various phase-shift degrees could easily be achieved by endowing the PM with a corresponding displacement in step 2. Figure 2(b) presents the spectra of PSBG-IIas with various phase-shift degrees; that is, π/2, π, and 3π/2, in accordance with the PM-displacements of Λ/4, Λ/2, and 3Λ/4, respectively. This result reveals that the PSBG-IIa spectrum could be flexibly tailored by adjusting the PM displacement. The wavelength tuning of the notch signal can also support the operation of a multichannel narrow-band filter [21].
Fig. 1 Step 1: Type-IIa fiber Bragg grating: (a) fabrication diagram; (b) spectra of transmission and reflection; Step 2: PSBG-IIa: (c) fabrication diagram; (d) spectra of transmission and reflection.
Fig. 2 (a) Transmission spectra of PSBG-IIa recorded by high-resolution OSA. Inset: phase-shifted notch partly extracted from entire spectrum (“T” and “λ” are abbreviations for “transmission” and “wavelength”, respectively; the same as below); (b) PSBG-IIas spectra with various phase-shift degrees. L represents the PM displacement in step 2.
3. Characteristics of PSBG-IIa
An attractive property of type-IIa Bragg gratings is the thermal resistance resulting from negative-index formation by means of the “breath” (compressing and then dilating) of the core network in the fiber [22]. It can be inferred that the notch will reserve its narrow-band pass competence at high temperatures. To confirm the high-temperature resistance capability, the PSBG-IIa thermal response was investigated by inserting it into a tube oven and heating it from room temperature to 600 °C [12]. As illustrated in Fig. 3(a), the wavelength of the notch red-shifted linearly with an increasing temperature, resulting in a temperature sensitivity of ~12.44 pm/°C over a large dynamic range. According to Fig. 3(a), the grating spectral shape changed only changed slightly during the heating process. In particular, even at 600 °C, the grating strength survived the thermal decay, maintaining the notch signal. Hence, the PSBG-IIa is capable of acting as a narrow-band pass filter, without compromise, in high-temperature environments.
Fig. 3 (a) Temperature response of notch signal. Inset: Transmission spectra of PSBG-IIa at various temperatures; (b) strain response of the notch signal. Inset: Spectra recorded at various strains by high-resolution OSA; (c) spectra recorded at various polarization states of input light source.
Immediately thereafter, the influence of the strain variation was tested on the grating by stretching it along the fiber axis. The dynamic range of spectrum was subjected to the displayed spectral width of high-resolution OSA, although, slight spectral variation of PSBG-IIa could be captured with a small range of strain changed from 0 to 240.9 με, as indicated in the inset of Fig. 3(b). The notch wavelength also shifted to longer wavelengths as the fiber tensioning progressed. Figure 3(b) illustrates the linear fitting between the notch wavelength and strain, reflecting a strain coefficient of 1.07 pm/με, which is similar to that of conventional FBGs.
Furthermore, the phase-shifted notch with high tunability offered the opportunity to study the birefringence induced by the high-dose exposure necessary to form a type-IIa grating. A polarization controller (PC) was linked to the light path to vary the polarization state of the input light source. By adjusting the PC, the notch spectrum was altered with a change in the polarization state, as indicated in Fig. 3(c). Only one resonant peak was present in the spectrum, as the polarization state of the input light coincided with the fast or slow axis of the grating device. Nevertheless, the resonance split into two peaks when the input light followed the polarization state between the fast and slow axes. The difference in the peak wavelengths was measured as 15.85 pm, corresponding to a birefringence of 1 × 10−5. This birefringence was primarily a result of the transversally asymmetrical change in the refractive index, caused by the relatively lengthy period of lateral irradiation of the UV light.
4. Laser applications
To demonstrate the filter capacity at high temperatures, the PSBG-IIa was introduced into a distributed Bragg reflector (DBR) cavity, as illustrated in Fig. 4. In this case, the Bragg reflectors were also negative-index gratings, with a reflectivity of 99%, inscribed at both ends of the M-5 fiber and a blank length of 19 mm. Each reflector was 3 mm long, and the total cavity length was 25 mm. The DBR fiber laser was pumped by a 980 nm laser with power of 100 mW via a 980-/1550 nm wavelength division multiplexer, enabling bidirectional laser emission at wavelengths among the 1550 nm band, depending on the wavelength selection of the Bragg reflectors. Thereafter, the backward output was logged by the high-resolution OSA using an optical polarization-dependent isolator, as well as a PC. First, the spectrum of the DBR laser without the PSBG-IIa was acquired. The wavelength spacing (Δλ) between the longitudinal modes in the cavity can be expressed as:
where n is the refractive index of the resonant cavity, L is the resonant cavity length, and is the center wavelength of the fiber laser. Thus, Δλ could be theoretically obtained as 33.03 pm. As depicted in Fig. 5(a), the 25 mm-long DBR laser without the PSBG-IIa exhibited three longitudinal modes simultaneously within the resonant spectrum. The effective mode space was 32 pm, which is in accordance with the theoretical results.Fig. 4 Experimental setup of DBR fiber laser, consisting of two type-IIa Bragg reflectors as well as a type-IIa phase-shifted Bragg grating filter.
To achieve single-longitudinal mode operation of the fiber laser, the PSBG-IIa, with a notch signal narrower than 7 pm, was inscribed directly into the DBR fiber laser cavity between the two reflectors, and the pass band complied with the reflectors. Figure 5(b) displays the spectrum outputted from the three-grating structure fiber laser. Only one laser signal existed, indicating that only the longitudinal mode located in the narrow notch band could achieve optical oscillation. Moreover, the fiber laser can operate in only one polarization state because the lasing window of the resonator is excessively narrow to ensure that two polarization modes exist simultaneously. Hence, the polarization mode satisfied by the major polarization axis operated at a low loss, and the orthogonal polarization mode was restrained.
The polarization extinction rate (PER) of the proposed fiber laser was studied, and is expressed as:
where Pmax and Pmin are the maximum and minimum output power values, respectively. The fiber laser output power, via the PC and a polarizer, was measured by a power meter. By rotating the PC, the maximum and minimum values of the laser output power could be obtained. When the pump power was set to 100 mW, the maximum output power was 1 μW, while the minimum output power was 1 nW. The output power, although, is not strong due to the short cavity structure and low absorption of the pump, the PER of the fiber laser was 30 dB according to Eq. (2).To verify the single polarization operation of the proposed fiber laser further, we injected the laser output into a photodetector and monitored the beat frequency signal through an RF spectrum analyzer. No beat frequency signal was observed in the range of 0 to 10 GHz. This clearly demonstrates that the PSBG-IIa provided the DBR fiber laser with a single longitudinal mode and polarization operation.
Subsequently, the three-grating DBR fiber laser was inserted into a tube oven to test its performance at high temperatures. As illustrated in Fig. 6(a), the wavelength of the DBR laser output exhibited a linear red-shift as the temperature increased, with a sensitivity of ~12.26 pm/°C. The laser output maintained single-longitudinal mode operation at various temperatures, as indicated in the inset of Fig. 6(a). Despite the increase in background noise with the increasing temperature, the laser output retained a high SNR of over 40 dB during the heating process. At 500 °C, the laser spectrum was monitored for 20 min. As indicated in Fig. 6(b), the result reveals that the lasing wavelength was stabilized in the vicinity of 1552.844 nm, with a standard deviation of 0.016 nm. Moreover, the laser output power remained at approximately −36.6 dBm without obvious degradation, indicating that the three-grating composite DBR fiber laser exhibited effective resistance at a high temperature, up to 500 °C. In particular, recorded by the high-resolution OSA, the laser output maintained single-longitudinal mode operation at 500 °C, as shown in the inset of Fig. 6(b). It could be indicated that the PSBG-IIa enabled a highly selective bandpass filter that can survive in high-temperature environments.
Fig. 6 (a) Lasing wavelength in response to temperature variation. Inset: DBR laser output spectrum at various temperatures; (b) long-term stability of laser wavelength and output power at 500 °C; Inset: the spectrum recorded by the high-resolution OSA at 500 °C.
5. Conclusions
In this study, a type-IIa phase-shifted Bragg grating was proposed for application as a high-temperature-resistant notch filter. Through designing the phase-shift amount, the filtering spectrum can be flexibly tailored. The mechanism of the negative-index change in the grating provides the notch signal with the capacity to be decay-free at 600 °C. Meanwhile, the birefringence of the notch signal was discovered, which was attributed to asymmetrical irradiation. A single-frequency DBR fiber laser can be achieved by inserting the PSBG-IIa into the cavity, where it can survive temperatures up to 500 °C. It is believed that the proposed fiber device will be conducive to extending the application of fiber sensors and lasers to high-temperature environments.
Funding
National Natural Science Foundation of China (NSFC No. 61775082); Pearl River Science and Technology Nova Program of Guangzhou City (No. 201610010151); Fundamental Research Funds for the Central Universities.
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