High-Resolution Interferometric Temperature Sensor Based on Two DFB Fiber Lasers with High-Temperature Monitoring Potential

Abstract

A high-resolution temperature sensor using the beat frequency measurement between the modes of two DFB fiber lasers is presented. The laser cavities are formed by the femtosecond inscription technique in a highly Er/Yb co-doped phosphosilicate fiber with low optical losses and compact design. The experimental results show a sensitivity of 1 GHz/°C, leading to a temperature resolution of 0.02 °C restricted by the thermistor used in the experiment. The maximum possible resolution determined by the laser linewidth is estimated as 2 × 10⁻⁶ °C. The operation of such a sensor at high temperatures (≈750 °C) with the possibility of further temperature increase is demonstrated. The combination of high resolution and broad temperature range makes the sensor attractive for various applications, especially in high-temperature monitoring.

1. Introduction

Optical fiber sensors are of great research and practical interest due to their attractive properties and the variety of measuring schemes, including photonic crystal fibers, fiber Bragg gratings (FBGs), different types of interferometers, etc. Such sensors are capable of monitoring various physical parameters including temperature, strain, refractive index and so on; see [1] for a review. If very high resolution is required, interferometric sensors are generally applied [2,3]. Their application fields include structural health monitoring, industrial process control and oceanographic monitoring [4,5,6], nano-biotechnology [7], micro-fluidic mechanics [8], and thermal drift measurement in electronic equipment [9].

Beat frequency shift measurement between the eigen-polarization modes of a single-frequency fiber laser (SFFL) [10] or between the laser under test and another narrow-linewidth (reference) laser [2] with a slightly different generation wavelength (heterodyning method) is an effective technique to fully exploit the characteristics of SFFL as a high-resolution sensor. The beat frequency signal can be easily measured using a radio frequency (RF) analyzer, whereas such a small absolute wavelength shift cannot be resolved using a conventional optical spectrum analyzer (OSA) or a scanning Fabry–Pérot interferometer (FPI).

To provide an overview of this method, we can mention that it was proposed in 1993 by Kim et al. in [11], where two polarization modes of the laser were used. The described polarimetric sensor was used to measure the lateral stress applied to a jacketed Nd-doped fiber laser. As already briefly noted, various schemes are used to obtain a beat frequency signal, which are described in more detail below. In [12], the beat signal between the longitudinal modes and the polarization modes of a single long cavity in a birefringent fiber was measured. Signal processing made it possible to simultaneously obtain information about deformation and temperature. The errors of the experimental measurement were within ±16.2 μstrain and ±1.9 °C. Using only one fiber laser and one demodulation system makes the system simpler, cost-effective, and portable. In [10], a polarimetric strain sensor based on the SFFL with distributed Bragg reflector (DBR) cavity generating two polarization modes demonstrated a sensitivity of −4.1 MHz/mstrain for linear strain and −0.37 MHz/(deg/cm) for torsional deformation.

Using two sources instead of one results in increased sensitivity of the sensor at the expense of its simplicity. One laser acts as a reference, while the second laser acts as the actual sensor. In [2], a high-resolution static strain sensor developed with a distributed feedback (DFB) fiber laser was presented. A reference FBG resonator was used for temperature compensation. The minimum static strain resolution was about 270 pε (10⁻⁶ mstrain). The beat frequency detection scheme with a ring SFFL and a tunable reference laser was proposed in [13]. An FBG as a sensing element was integrated into the scheme via a circulator embedded in the ring laser cavity. The typical response of the system is about 1.3 GHz/°C, with nominal temperature measurement resolution of 0.0023 °C being achieved. A version of the scheme based on a single-frequency ring fiber laser was proposed in [14]. Similar to [13], the cavity FBG was used as a sensing element and the commercial tunable laser generating near 1.5 µm was used as a reference. The SFFL works as a temperature sensor over the range of 3–85 °C with the corresponding sensitivity of 1.83 GHz/°C (14.74 pm/°C). The experimental resolution was about 5 × 10⁻³ °C (at a theoretical limit of ~5 × 10⁻⁶ °C) because of the beat frequency jitter.

In [15], a high-resolution temperature sensor using the beat frequency between the longitudinal modes of twin single-mode DFB fiber lasers (reference and sensing) was presented. The lasers were made by means of femtosecond laser inscription of π-shifted FBGs in a thulium-doped fiber. The experimental results showed a sensitivity of 1.9 GHz/°C, leading to an accuracy of 0.0007 °C defined by the DFB laser linewidth. The sensing scheme of [15] has the best sensitivity among heterodyne beat schemes and comparable performance with other high-resolution fiber temperature sensors.

The field of applications where it is important to perform high temperature (>100 °C) measurement with high accuracy is extensively growing. For example, polymer synthesis is a multi-stage process requiring a very precise heating rate up to 300–500 °C [16,17]. In chemical synthesis in the pharmaceutical industry, accuracy over a wide temperature range is a critical parameter [18]. Also, temperature accuracy in the range of ~100 °C is very important when manufacturing microelectronics using photolithography, since there is a direct correlation between the uniformity of temperature on the surface of the heating plate and the uniformity of the critical size of elements on the silicon wafer. A temperature range of just a few hundredths of a °C on the wafer can lead to an unacceptable change in the size of transistors, which makes such chips inoperative [19,20]. This list can be continued with a calibration of thermocouples [21] and furnaces used for the investigation of various materials [22] and heat treatment of alloys [23] and other industrial applications. For high temperature monitoring in harsh environments in such areas as metallurgy, fuel production, and aerospace research, specialized fiber-optic high-temperature sensors are used [24].

According to the temperature measurement principle, high-temperature fiber-optic sensors can be divided into black body radiation sensors, fluorescent sensors based on various rare earth materials, interferometric sensor structures in silica and crystal fibers, FBGs in silica-based and crystal fibers, and Raman distributed temperature sensors (DTS) [24]. Among the many types listed above, interferometric sensors provide the best temperature resolution, but for high-temperature measurements passive sensors are more commonly used. The interferometric sensors can be categorized depending on the type of interferometer and fiber used, as well as the type of modes involved in the light interference process (coupling of core modes, core and cladding modes, etc.). The optical path difference (OPD) in the interferometer varies depending on the outside temperature, so temperature measurement can be achieved by calculating the OPD value from the interference spectrum. It should be noted that high-resolution fiber-optic temperature sensors operating under high temperature conditions have not been described in the literature, albeit fiber laser with linear cavity based on an FBG fabricated by femtosecond laser is shown to be used as a high temperature sensor [25]. The fiber laser can work stably up to temperatures as high as 1000 °C (after FBG annealing) and its temperature sensitivity is about 15.9 pm/°C; however, the temperature resolution is rather low in this scheme, since the broadband FBG determines the width of the laser line and the temperature resolution as a result.

In this paper, narrowband (single-frequency) DFB fiber lasers fabricated by femtosecond technology in a specialty highly Er/Yb co-doped phosphosilicate fiber [26] providing compact design are applied for high-resolution measurements at high temperatures, for the first time to our knowledge. Two identical lasers generating near 1535 nm with nearly the same characteristics (linewidth of ~2 kHz, differential efficiency of ~4.4%) are used in a heterodyning temperature sensor based on the beat frequency measurement. It demonstrates a sensitivity of 1 GHz/°C and its linewidth-limited resolution is estimated as ~2 × 10⁻⁶ °C at room temperatures around 24 °C, which allows us to classify this sensor as highly sensitive. It is also shown that the operating temperature range of the presented lasers can be extended up to ~750 °C (and potentially higher with annealing), allowing us to use them in high-temperature measurements.

2. Experiments and Results

2.1. DFB Laser

As known, the output power of Er fiber lasers is only ~0.1 mW [27,28], and for many applications it is necessary to use amplifiers, which leads to an increase in the form factor of the device and complicates its integration into the overall circuit. The use of the fiber co-doped with Er and Yb ions as an active medium of a DFB laser makes it possible to obtain higher output power in the range of 1.5–1.6 µm due to the direct transfer of excitation energy from Yb³⁺ to Er³⁺. To fabricate compact DFB lasers, we use a specialty highly Er/Yb co-doped phosphosilicate fiber, the manufacturing process and characteristics of which are described in detail in [26]. The absorption coefficient for the used fiber is about 1500 dB/m at the pump wavelength (~980 nm) and 60 dB/m at the generation wavelength (~1535 nm).

The cavity of each DFB lasers representing an FBG with a π-phase shift in the center of the structure has been inscribed using a femtosecond point-by-point technique [27] with the same parameters for both FBGs and a length of about 30 mm for each. The experimental scheme for the characterization of the DFB laser based on the π-shifted FBG is presented in Figure 1.

The fiber laser was pumped through a 980/1500 nm WDM by a single-mode laser diode with a wavelength of ~980 nm and an output power up to 400 mW. The laser output radiation counter-propagating with the pump also passed the WDM and isolator, minimizing the influence of back reflection from the components and was divided into 2 measuring channels by the 1 × 2 splitter. The generation wavelength and output power were measured by a Yokogawa AQ6370 optical spectrum analyzer (OSA) with a resolution of 20 pm. The relative intensity noise (RIN) was obtained using an Agilent N9010A radio frequency (RF) analyzer and a 5 GHz Thorlabs DET08CFC photodiode. The measured optical spectra and output powers of two DFB lasers presented in Figure 2 show that they are nearly identical. Both lasers generate near 1535.2 nm, and their signal-to-noise ratio is about 70 dB (Figure 2a,b). The lasing threshold is observed at a pump power of ≈150 mW for both lasers, then the output power grows linearly with a slightly different slope of around 4.4% (Figure 2c). The maximum output power from the signal port of the WDM at a pump power of 400 mW amounts to 10 and 11 mW (for lasers #1 and #2, respectively). The laser cavity is symmetrical; thus, the same power is registered in the direction co-propagating with the pump.

The relative intensity noise measured at the maximum output power of the lasers is shown in Figure 3a,b, whereas Figure 3c shows the beat frequency signal of lasers #1 and #2 obtained by the heterodyning method [29], with an RF analyzer allowing for the high-precision laser linewidth detection. The value of beat signal spectral width is about 40 kHz at a level of −20 dB, which corresponds to the laser linewidth (at −3 dB level) of ~2 kHz. At the same time, the value of relative intensity noise appears to be lower than −100 dB/Hz at the resonance frequency of 0.54 and 0.6 MHz for the first and second lasers, respectively (Figure 3a,b), which corresponds to the standard value for commercial lasers of this type.

2.2. Temperature Measurements

The lasers described above were used to implement an interferometric temperature measurement scheme, as illustrated in Figure 4.

The interfering DFB lasers were pumped independently by two laser diodes with a wavelength of 980 nm through WDMs. The laser output radiation counter-propagating with the pump passed the WDMs and isolators for further mixing on a 2 × 2 coupler. The mixed signal was divided into 2 measuring channels for beat signal and optical spectra registration as shown in Figure 1 with the measuring equipment described above. The cavities of the DFB lasers were placed in temperature controller units. The average beat frequency value over a measurement time of 100 s at room temperature (≈23.7 °C) was about 212 MHz; see inset in Figure 5a. The beat peak fluctuations are mainly in the range of 170–260 MHz. Standard deviation is estimated as 20 MHz.

For the temperature sensing experiment, the first (reference) laser remained at room temperature and the second one was heated. The dependence of the beat frequency on temperature in the range of 23.7–25.7 °C is presented in Figure 5. The experimental data are well approximated by linear fit with a slope of about 1 GHz/°C. Since the laser linewidth is about 2 kHz, the obtained sensitivity value corresponds to the theoretical limit for a resolution of ~2 × 10⁻⁶ °C. Taking into account the frequency jitter with a standard deviation of 20 MHz determined by the thermistor, the resolution in the experiment is only 0.02 °C.

It should be noted that for both lasers, a single-frequency and single-polarization generation regime is observed in the entire measurement range that is confirmed by a single beating peak shifted with a temperature growth in the whole frequency range of the RF analyzer. The variation in the beat signal peak based on the temperature at maximum pump power is 5–10 dB (Figure 5b), which may be due to convection inside the oven affecting the position of the fiber.

To determine the operating temperature range of the presented DFB lasers the following measurements were carried out: DFB laser was heated up in the oven by 750 °C, at which point the generation wavelength shift was registered by OSA; see Figure 6. For high-temperature experiments we used an LOIP LF-50/500–1200 tubular furnace with instability of temperature maintenance within the range of ±10 °C. The measured generation wavelength shift is demonstrated in Figure 7.

When the temperature reached ≈775 °C the generation power decreased by ≈37 dBm, at which point the generation wavelength shifted from 1535.5 to 1541 nm (the red shift amounts to ≈5.5 nm). In the inset in Figure 7 the generation spectra under a further temperature increase up to 1000 °C (green curve) are shown, at which point significant power degradation is observed due to the decrease in the gain of the media conditioned by thermal quenching [30]. One of the origins of quenching is the excitation of phonon modes, which transfer energy to excited ions, leading to nonradiative transitions (so-called nonradiative relaxation). Metastable level lifetimes are reduced at increasing temperatures, reducing the population inversion. Nevertheless, the signal is visible and may be amplified for sensing almost up to ~1000 °C.

It should be noted that under the heating, the π-shifted FBGs do not deteriorate, which was confirmed experimentally: after cooling, the spectrum of the laser was restored. So, the DFB-laser based sensor may be repeatedly used for high-temperature monitoring.

2.3. Temperature Test of DFB-Lasers Based on Phase-Shifted FBGs Inscribed by Different Techniques

In addition, the effect of moderate heating (up to 330 °C) on the characteristics of the DFB lasers with the phase-shifted FBG cavity inscribed using the UV holographic technique and the fs technique have been compared. The FBGs were placed into the oven and heated up to 330–350 °C, which is far from the degradation temperature of the fs-inscribed DFB laser. The heating from room temperature up to 330 °C was achieved in 30 min; see Figure 8a (black). In the case of UV-inscribed FBG, generation was interrupted at ≈330 °C, as shown in Figure 8a (blue). This is due to the fact that high temperature causes annealing of defects created by UV radiation in the germanosilicate fiber. In turn, this leads to a decrease in the induced refractive index, a weakening of the grating reflectivity and a decrease in the Q-factor of the resonator. As a result of the annealing, the transmittance of the FBG at the resonant wavelength has decreased from −29 dB to −22.5 dB, as seen in Figure 8b.

The DFB-laser based on the fs-inscribed FBG exhibits laser generation in all temperature ranges and operates at the highest temperature for more than 4 h, as seen in Figure 9a, as expected. The transmittance of the grating changed insufficiently, as shown in Figure 9b. The observed difference in resonant wavelengths is due to the fact that the FBG was fixed in the oven with some tension.

3. Discussion and Conclusions

As can be seen from the literature, some sensing schemes may be more sensitive than the one presented in this paper [3,14,15,31], as shown in

Table 1. Performance comparison of wavelength encoding temperature sensors.

Reference	Sensitivity	Resolution	Operating Temperature	Sensing Scheme 4
This paper	7.85 pm/°C	2 × 10−6 °C *	20−750 °C	Beating between twin single longitudinal mode DFB lasers
[3]	84.6 pm/°C	6 × 10−4 °C	20−100 °C	Fabry–Pérot silicon pillar tip
[14]	14.74 pm/°C	0.005 °C	3−85 °C	Beating between SFFL and reference laser
[15]	23.4 pm/°C	7 × 10−4 °C	15−35 °C	Beating between twin single longitudinal mode DFB lasers
[31]	20 pm/°C	0.58 °C	19−520 °C	Miniaturized fiber taper reflective interferometer

* Potential value.

(see also Table 1 in [15]), or have comparable sensitivity [13,32]. However, the corresponding setups are more difficult in implementation (usually involving complex components) and represent a low signal-to-noise ratio or large linewidth, leading to poor resolution. Moreover, these works are mainly devoted to measurements at temperatures up to 100 °C.

The sensing scheme presented here has comparatively good sensitivity (1 GHz/°C or 7.85 pm/°C) among heterodyne beat schemes and has comparable performance with that of high-resolution fiber temperature sensors. As mentioned above, the resolution of the scheme could be sufficiently (by four orders) improved by improving thermal element stability, namely using another more stable device, while the linewidth of the lasers is quite narrow for obtaining high resolution. Moreover, it may be further reduced by means of fs-inscribed artificial Rayleigh reflectors (ARR) attached to the DFB laser [33]. At the same time, fs-inscribed DFB lasers (and ARR as well) are stable at heating (up to ~1000 °C) and may be repeatedly used for high-temperature monitoring, in contrast to traditional UV-inscribed DFB lasers, which already start degrading at ≈330 °C.

In conclusion, DFB lasers with a length of 30 mm based on highly Er/Yb co-doped phosphosilicate optical fiber with a linewidth of ~2 kHz and differential efficiency of about 4.4% have been developed and implemented for high-temperature sensing.

A high-resolution temperature sensor was realized using twin DFB lasers in a heterodyning scheme, implying measurements of the beat frequency between them with a sensitivity of 1 GHz/°C and linewidth-limited potential resolution of 2 × 10⁻⁶ °C, which allows us to consider the presented sensor as highly sensitive and high-resolution. The resolution of 0.02 °C obtained in the experiment was restricted by the thermistor used in the experiment as an object of measurements.

It has been demonstrated that proposed lasers based on femtosecond-inscribed phase-shifted FBGs are resistant to heating in terms of their characteristics up to ≈750 °C (and higher with signal amplification), which provides an opportunity to combine both high-resolution and high-temperature measurements. Such sensors are quite important in applications such as the synthesis of polymer and other chemicals, microelectronics, aerospace, metallurgy, fuel and power production, and so on; see [24] for more details. In addition to temperature sensing, strain or bending measurement could also be performed using the setup presented here with proper alterations in the scheme for applications in various fields [34,35].