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We report a wavelength-swept fiber laser with high speed and wide tuning range, and its application to fiber sensors. The laser is based on the dispersion tuning technique, which does not require any optical tunable filter in the laser cavity. By directly modulating the semiconductor amplifier and adjusting the dispersion in the cavity, a wide wavelength tuning range of 178.7 nm and a fast tuning rate of over 200 kHz are obtained. The wavelength-swept laser source is applied to a dynamic fiber Bragg grating sensing system. Dynamic measurement of a 150 Hz sinusoidal strain is demonstrated with a measuring speed as fast as 40 kHz.
©2009 Optical Society of America
1. Introduction
Wavelength-tunable lasers are versatile both in telecom and sensing applications. A wide tuning range and a fast tuning (sweep) rate of the laser are usefully required in order to enhance the spatial resolution, the measurement range, as well as the measurement speed for efficient dynamic sensing. Many different kinds of wide and fast wavelength-swept lasers have been proposed, in which most of the schemes consist of a wide gain medium and a fast tunable optical filter in the laser cavity. The wide gain medium is typically a semiconductor optical amplifier (SOA) or an erbium-doped fiber amplifier (EDFA). EDFA-based lasers can be tunable over 100nm [1], but its wavelength band is limited to 1550nm. The SOA-based lasers can work at many different wavelength bands with wide gain bandwidth over 100nm. For the sweeping speed there are two limiting factors which are the sweep speed of the optical filters and the photon lifetime in the laser cavity, which is inversely proportional to the cavity length. There have been several fast tunable optical filters including piezo-transducer (PZT)-based tunable Fabry-Perot filters (FFP) [1], and polygonal mirror scanners [2]. They are basically mechanically tunable filters and the mechanical sweeping speed is normally limited to a few tens of kHz. FFPs have been shown to be able to be swept much faster at a few hundreds of kHz by utilizing the resonance of the specially-designed PZT [3]. Also, the cavity length should be kept as short as possible for fast tuning. Therefore, the SOA-based lasers are more advantageous than the EDFA-based lasers since the cavity can be shorter. As a different approach to this issue, the Fourier-domain mode locking (FDML) has been proposed recently, in which the sweeping time is set to be equal to one (or integer times of) round trip time of the cavity [4]. With the FDML scheme, a 100 nm tuning range and a 370 kHz sweep rate have been achieved, but such high-speed FFP is not readily available.
We recently proposed a different type of fast and wide tuning range wavelength-swept fiber laser using the dispersion tuning scheme [5, 6], which is based on the active mode locking in the dispersive laser cavity. Since this technique does not require any optical tunable filters, the laser can be operated at a wide tuning range and a fast tuning speed. With the dispersion tuning, 105 nm tuning range and 200 kHz sweeping rate have been previously achieved [5].
Fiber-optic sensors are attractive for health monitoring in many fields such as buildings, ships and airplanes, serving as smart structures and smart materials [7]. In particular, Fiber Bragg Grating (FBG) sensors have been widely studied in various methods due to their unique characteristics for sensor applications [8-11]. In basic FBG sensor systems, the Bragg wavelength shift of FBG is monitored with broadband sources from a light emitting diodes (LED) or an EDFA. However, these schemes usually suffer from a relatively low signal-to-noise-ratio (S/N) of sensor signals and a slow measurement speed. In order to obtain higher sensor signal power, a wavelength-tunable laser source with fast tuning speed is always required [12-15].
In this paper, we improve the tuning characteristics of the dispersion tuning-based wavelength-swept fiber laser at 1.5 μm bands, and apply it to a dynamic FBG sensing. A very wide tuning range exceeding 170 nm with a fast tuning rate over 200 kHz has been realized. With the swept laser, dynamic measurement of a 150 Hz sinusoidal strain is successfully demonstrated with a measuring speed as fast as 40 kHz. The results show that the laser is suitable for dynamic FBG sensing with multiplexed FBG array, and also suitable for other sensing applications such as the optical coherence tomography (OCT) systems.
2. Dispersion tuning-based wavelength-swept fiber laser
2.1. Principle of dispersion tuning
The free-spectral range (FSR) of the laser cavity F can be expressed as,
where L is the cavity length, n is the refractive index in the cavity, and c is the speed of light in vacuum. When the cavity contains chromatic dispersion, the FSR is a function of wavelength λ as shown in Fig. 1. Denoting the FSR at a wavelength λ as F, and ignoring higher order dispersion, the wavelength λ and the FSR F has the following relation,
where n is the refractive index at λ, and D is the dispersion parameter. Active mode-locking is a technique to generate short pulse trains by applying a modulation to the laser cavity. For stable active mode locking, the modulation frequency fm applied to the cavity has to match with an integer (N) times of the FSR (= N · F), where N is the order of harmonic mode locking. That is, when we apply a modulation at fm to the dispersive cavity, the laser is forced to operate at the wavelength λm to meet the harmonic mode-locking condition, which can be expressed as,
where f m0 = N · F. Therefore the lasing wavelength can be tuned by changing the modulation frequency. This tuning process is depicted in Fig. 1 which is also known as dispersion tuning [16].
Wavelength tuning range Δλmax is determined by the gain bandwidth of the gain medium and lasing at the adjacent harmonic mode, (N-1)-th or (N+1)-th mode. It happens when the change of modulation frequency exceeds one FSR. From Eq. 3, Δλmax can be expressed as,
which indicates smaller f m0,∣D∣,L is needed for wider tuning range.
The pulse of the mode–locked fiber laser in dispersive cavity is expressed with a chirped Gaussian pulse and its bandwidth δω is given by the following equation [17],
where M is the modulation depth. Eq. 5 indicates equation suggests that smaller f m0 and larger ∣D∣ and L are needed for narrower instantaneous bandwidth. However, from Eq. 3, smaller f m0 increases the instability of lasing wavelength and causes linewidth broadening which is common in the following experiments. Thus f m0 should be as high as the linewidth broadening according to Eq. 5. Since the tuning rate is inversely proportional to the photon lifetime (∝ cavity length) of conventional tunable fiber laser, a faster tuning rate can be obtained with a shorter cavity length.
An advantage of applying this method is that wavelength tunable optical filters such as fiber Fabry-Perot filters are unnecessary, unlike the conventional methods. Thus, higher tuning rate is expected in this method.
Fig. 1. Concept of the dispersion tuning. The lasing wavelength is tuned as the mode-locking freuency changes.
2.2. Construction and characteristics
Figure 2 shows the schematic setup of the dispersion tuning-based wavelength-swept fiber laser. The laser cavity is constructed with a ring configuration and all the devices adopted are pig-tailed with single-mode fibers. The polarization-independent semiconductor optical amplifier (SOA) module (Covega SOA1013) is used as the gain medium of laser. The SOA has a 3-dB gain bandwidth of 85.7 nm and a 10-dB gain bandwidth of 170.91 nm with an applied current of 147 mA. Active mode-locking is achieved by directly modulating the injection current to the SOA with the RF signal from a RF synthesizer (Agilent N5181A), which can effectively reduce the intracavity loss and elimination of an external intensity modulator. The triangular or ramp waveform from a function generator (FG, Agilent 33250A) is used to modulate the RF synthesizer for lasing wavelength tuning. In order to provide the chromatic dispersion in the laser cavity needed for the wavelength tuning, we insert a 100-m-long dispersion compensating fiber (DCF) with a dispersion parameter of -90 ps/nm/km at λ = 1550 nm. An isolator in the SOA module ensures unidirectional light propagation in the laser cavity. Finally 10 % of the light in the laser cavity is output via a 9:1 coupler.
In our previous work, we used a polarization-dependent SOA at 1.3 μm wavelength band region [5]. Therefore, the laser has to be a sigma-laser configuration using polarization maintaining fibers (PMF), and its output spectrum was jaggy due to the mode-coupling in the PMF. In this work, we used the polarization-independent SOA so that the laser can be a simple ring configuration using standard single-mode fibers. Also, we can optimize the length of the DCF in the cavity and the mode-locking frequency in order to achieve much wider tuning range.
We set the mode-locking frequency at around 410 MHz which is determined by the RF modulation characteristics of the adopted SOA. Figure 3 shows the change of the lasing spectra and the lasing wavelength as the mode-locking frequency is adjusted manually. The results show that the lasing wavelength shifts almost linearly towards the longer wavelength as the mode-locking frequency increases, which is consistent with Eq. 3. The tuning sensitivity is 138.1 nm / MHz and the static tuning range is 178.7 nm. The tuning sensitivity calculated by Eq. 2 is 133.3 nm / MHz, which is in good agreement with the experimental results. Owing to the small loss in the laser cavity, we can achieve much wider tuning range than the 3-dB bandwidth of the SOA. The output power is about 1.3 dBm and the instantaneous linewidth is about 1.1 nm when the lasing wavelength is 1540 nm. As discussed in the previous section, an optimum f m0 can be obtained in order to give the narrow linewidth. In our experiment, we have confirmed that the linewidth can be narrower when f m0 increases. We have realized the instantaneous linewidth of 0.2 nm by using larger f m0 at 1.0 GHz [5]. Therefore, a narrower linewidth is expected by increasing f m0 with a proper SOA.
Fig. 3. The static characteristics of the swept laser (a): optical spectra and (b): lasing wavelength as a function of the mode-locking frequency
The mode-locking frequency is set to be around 410 MHz in order to modulate linearly with the triangular waveform and sweep the wavelength linearly as shown in Fig. 4. We use the external-FM function of the RF synthesizer to modulate the mode-locking frequency, where the triangular signal from the function generator is used as the external-FM signal. The triangular waveform consists of two scan areas, up-scan and down-scan. In the up-scan area, the lasing wavelength shifts toward longer wavelength. Figure 5(a) shows the peak-hold spectra obtained by the optical spectrum analyzer (Anritsu, AQ6317). Figure 5(b) shows the temporal waveforms when the mode-locking frequency is modulated at different tuning rates of 200 Hz, 2 kHz, 20 kHz, 60 kHz, 100 kHz, and 200 kHz. A dynamic tuning range of over 120 nm is achieved at 20 kHz tuning rate. It is observed that the intensity of the peak-hold spectra decreases when the tuning rate increases owing to the integral time of the peak-hold function of the optical spectrum analyzer. When the scanning speed is below the inverse of the integral time, the peak value is refreshed at every scan. Therefore the intensity is decreased while the optical power level is kept as shown in Fig. 5(b). On the other hand, when the scanning speed is fast the peak value becomes constant regardless of the scanning speed as shown in Fig. 5(a). The limitation of the tuning range at higher scan rate originates from the smaller gain at the edge of gain bandwidth of the SOA. Undulations in the peak-hold spectra in the longer wavelength region are observed as the sweep speed increases due to the difference between the waveforms in the up-scan and down-scan as shown in Fig. 5(b). The difference between the up-scan and the down-scan is believed to be originated from the nonlinear effect of the SOA [18]. An output power of -1.95 dBm is measured when this laser is swept at a tuning rate of 200 kHz.
Fig. 5. The dynamic characteristics of the swept laser. (a): the peak-hold spectra and (b): the temporal waveform when the mode locking frequency is modulated by triangular waveform
The swept laser is found to be applicable to the dynamic measurement in FBG sensing system with a large number of FBGs. The tuning rate of the laser is determined by the photon lifetime only since no optical tunable filter is needed in the cavity. Note that higher tuning rate is possible by reducing the cavity length with a higher dispersive element.
3. Application to FBG sensor
3.1. Principle and experimental setup
In this session, we apply the proposed wavelength-swept laser to a FBG sensing system. The experimental setup of our proposed FBG sensor is shown in Fig. 6. The multiplexed FBG array (Fujikura) consists of different Bragg wavelengths, where FBG1 = 1525 nm, FBG2 = 1540 nm, FBG3 = 1550 nm, and FBG4 = 1560 nm. All FBGs are with more than 90 % reflectivity including fusion splicing loss. The reflected light from the FBG array is launched to a photodiode (New Focus 1611) via a circulator. The sensor signal is then A/D converted with the trigger signal from the function generator. In order to control the A/D-converter (NI PCI6251A) and calculate the change of the Bragg wavelength, LabVIEW (National Instruments) is used. As shown in Fig. 7(a), when the optical source is swept, the laser output light scans each FBG. Only the light corresponding to each FBG’s Bragg wavelength is reflected and converted to an electrical signal as shown in Fig. 7(b). In this system, we interrogate the locations of each pulse in the temporal waveform and calculate the relative wavelength using the reference FBG. When the reference FBG is No. 3 as shown in Fig. 7, the relative wavelength Δλ = ∣λ 2 − λ 3∣ is estimated from Δt = ∣t 2 − t 3∣ using the following equation,
Fig. 7. Interrogation process of our FBG sensor (a): optical spectrum and (b): detected temporal waveform
where Δλ is the relative wavelength from the reference FBG, Δt is the relative time from the reference FBG, Ss is the tuning rate (Hz) of the swept source, and Sr is the tuning range (nm) of the swept source. Note that the Δt (Δλ) changes in proportion to the strain added in the FBG.
Instead of calculating Δt from the peak values of the waveform in Fig. 7(b), we adopt the differentiated waveform in order to reduce the fluctuations by noises and obtain accurate relative wavelengths. The waveform is initially filtered by a low pass filter (LPF, type II Chebyshev filter) for noise reduction and differentiates against time.
3.2. Results
The temporal waveform of the reflected light from the FBG array and its differentiated waveform after LPF at 500 kHz are shown in Fig. 8 with the laser sweeps at 40 kHz. We use the down-scan for interrogation of the FBG array by the negative ramp waveform as shown in Fig. 8(a). The cutoff frequency of 500 kHz is chosen experimentally in consideration of sensor signal spectrum. Form the figure, we observe that the multiple traces of the four temporal waveforms are broadened and overlapped at higher scan rates. It is because of the insufficient speed of the A/D-converter as well as the broadened instantaneous linewidth of the laser at high scan rate, which are under investigation and will be reported. Table 1 summarized the static properties of the system as well as averages and standard deviations of the relative wavelength from the reference FBG3 measured for 5 seconds.
Figure 9 shows the change of relative wavelength when the FBG2 is stretched manually. It is observed that the relative wavelength changes linearly as the strain is added, which suggests that this sensor system works correctly. The slope of the linear-fitting is 0.99 pm / (μ strain).
Table 1. Static properties of the system
Figure 10 shows the results of the dynamic sensing when a periodical strain is added to FBG 2. The dynamic strain is applied with a PZT stage (PI) driven by a sinusoidal waveform from a RF function generator. When the frequency of the strain changes abruptly from 1 Hz to 10 Hz, it is confirmed that this sensor can capture the change of the frequency as shown in Fig. 10(a) (Media 1). Figure 10(b) shows that the dynamic strain can be measured correctly when the strain is added with a 150 Hz sinusoidal vibration. The resolution of this sensor is limited by the FBGs used since our swept laser source has wider instantaneous linewidth than the FBGs. The results indicate that this system is capable to work accurately and measure transient distortion at a high measurement rate.
Fig. 10. Dynamic response of the system (a): Abrupt change of applied sinusoidal strain from 1 Hz to 10 Hz (Media 1) (b): The FFT spectrum when the 150 Hz sinusoidal strain is added.
4. Conclusion
A fast and wide tuning range wavelength-swept fiber laser has been demonstrated. With dispersion-tuning in the laser cavity, a wide wavelength tuning range of 178.7 nm and a fast tuning rate of 200 kHz are obtained. The laser is applied to a dynamic FBG sensing system, and a dynamic measurement of a 150 Hz sinusoidal strain has been successfully achieved with a fast measuring speed of 40 kHz. It is expected that the tuning range and speed of the laser can be improved by shortening the laser cavity, adopting a higher dispersive medium in the cavity, as well as adopting a higher performance SOA. Also, the measurement speed of the sensing system can be further increased by using higher-speed A/D-converter. The results show that such wavelength-swept fiber laser is promising for efficient dynamic FBG sensing systems with a high accuracy and fast measuring speed.
Acknowledgements
The authors wish to thank Dr. Koji Omichi of Fujikura for providing part of the FBGs used in the experiments.
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