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Abstract
In this work, we report the development of an external-cavity wavelength-swept amplified spontaneous emission (ASE) source with high output power and high tuning speed based on an efficient electro-optic effect of beam deflection. The wavelength-swept ASE source is capable of delivering stable output power with averaged intensity of 100 mW in a wide spectrum tuning range of over 80 nm around the wavelength of 1550 nm. The light source will have important applications in optical communications, biomedical imaging, spectral analysis and sensing.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Compact agile wavelength-swept light sources with high power are critical components for many applications, such as telecommunications, optical coherence tomographic imaging, display and remote sensing [1–11]. Currently, there is a technical challenge to implement high-power and high-speed continuous laser tuning with less fluctuations of intensity and wavelength in a relatively wide spectral range. External-cavity laser diodes with wavelength-selective elements are capable of delivering laser output with narrow spectral linewidth in a wide tuning range. However, most of these lasers exploit mechanical scanning methods which suffer from slow speed and low precision of wavelength selection [12,13].
Electro-optic (EO) components offer the most fast and precise way to control the state of light by modifying its polarization, intensity or phase, and thus have great applications in optical communications, lasers and sensing [14]. EO effects have also been used to deflect the direction of light propagation by utilizing prisms of electro-optic crystals which exhibit the change of refractive index under applied electric field. Although such beam deflectors possess fast time response, they can only deflect light beam to a small angle with limited number of resolvable spots [15]. Recently, KTa1−xNbxO3 (KTN) crystals have been studied for many potential applications including high-speed beam deflectors [16–20], electro-optic modulators [21,22], photorefractive light needles [23], scale-free optics and diffractionless waves [24–26]. Due to its large EO coefficients near phase transition temperature, the KTN crystal can deflect light beam to a relatively large angle under low driving voltage, which is very useful for spectral tuning and wavelength selection applications.
Yuzo Sasaki et al [27] have recently demonstrated a new swept source at 1300 nm for the application of optical coherence tomography incorporating an electro-optic KTN crystal. Although they have demonstrated a repetition swept rate of 200 kHz, the output power is only about 20 mW. In this paper, we report the fabrication of a high-speed wavelength-swept ASE source with the maximum output power of exceeding 100 mW. The light source is capable of delivering very stable output power in a wide spectral range of 80 nm of continuous tuning around the wavelength of 1550 nm.
2. Wavelength-swept ASE source design
2.1 Wavelength tuning principle
The high-power efficient wavelength-swept ASE source is based on the principle of grating diffraction and the space-charge-controlled EO effect [16–20]. A reflective ruled diffraction grating is used to select the output wavelength. The light source output wavelength as a function of the beam angle incident to the grating can be expressed as:
where θ0 is the initial incidence angle and a is structural parameter of the grating. When the value of Δθ is small (around 2 °), the relationship of λ and Δθ tends to be linear.To achieve no-mechanical high-speed tuning of output wavelength, we use an electro-optic deflection device based on pure KTN crystal to regulate the increment of the angle incident to the grating. Based on the space-charge-controlled EO effect, the large beam deflection angle from the KTN crystal can be expressed as [18, 19]:
where n0 is the original refractive index, g11 is the EO coefficient, e is the elementary electric charge, L is the interaction length, x is the distance from the cathode, d is the thickness of the KTN, ε is the permittivity, V is the bias applied voltage, and N is the density of electrons which are injected from the cathode and stored at the electron traps of the crystal. Equation (2) is an approximate formula under the condition that is sufficiently small compared to 1. We take the absolute value of eN because the origin of the charge is the injected electrons, and eN is negative [19].When the beam is incident to the center of the crystal, the deflection angle is given by:
thus, .Combining Eq. (1) and Eq. (3), the output wavelength can be expressed as:
where c is a constant, and V0 is the corresponding voltage when the angle incident to grating is θ0.2.2 Wavelength-swept ASE source system
Figure 1 illustrates a schematic of the external-cavity wavelength-swept ASE source system based on an efficient booster optical amplifier (BOA-6323, Covega) and a fast EO wavelength-selection unit. The light emitted from the BOA is polarized along x-axis and coupled into a polarization-maintaining single-mode optical fiber to the high-speed KTN electro-optic scanner which deflects the light beam onto a reflection grating (600 /mm, 1600 nm blaze, GR13-0616, Thorlabs) in a simple Littrow configuration. According to the principle of grating diffraction, the diffracted beam with a certain wavelength can be back-reflected into the direction of the incident beam when a collimated beam of multiple wavelengths is incident onto the Littrow grating. The wavelength of the back-diffracted beam can be changed or scanned depending on the incident angle. The reflected beam is then re-coupled into the light source system through the EO deflector and the polarization-maintained single-mode optical fiber to the BOA chip which amplifies the reflected beam with a great gain. Prisms used in these setup are anamorphic prisms (Thorlabs, PS872-C).
Fig. 1 Schematic of the high-power wavelength-swept ASE source design. A: Littrow configuration with a reflection-type grating; B: three-prism beam expender; C: cylindrical concave lens; D: KTN crystal placed on a thermoelectric controller (TEC); E: polarizer; F: polarization-maintaining single-mode fiber with GRIN lens; G: a semiconductor optical amplifier; H: optical detector (or optical spectrum analyzer).
High-quality KTN crystal used in our study is grown by top-seeded solution growth technique, which has an abnormal large relative permittivity in the vicinity of Curie temperature (TC). For obtaining a stable and efficient performance, we set the KTN scanner operation temperature to 50 °C, which is just two degrees above TC. Based on the space-charge-controlled electro-optic effect, the refractive index of the KTN crystal along the x-axis is regulated by applied field. However, the KTN deflector also has a lens effect which is a by-product of the space charge field. The output beam will converge in the direction of applied electric field and deform to an elliptic shape. To balance this offset, a cylindrical concave lens is placed behind the end of the KTN crystal to collimate the beam, and shape the beam into a circular configuration [19,28]. This ensures that the beam incident to the grating is collimation beam.
In addition, we have observed that the output beam intensity is much stable and the beam linewidth is less than 0.5 nm throughout the wavelength tuning range when prisms are used to elongate the illuminating area of the collimated beam across the grating grooves. Instead of using a pair of prisms, we use three prisms to expand the beam. With such configuration, the grating surface is aligned perpendicularly to the z-axis, making the laser cavity much compact and the optical alignment much simple.
3. Wavelength-swept ASE source performance
Upon the construction of the wavelength-swept ASE source, we characterize the output beam performance including wavelength tuning capability, linewidth, output power as well as wavelength-swept time response. We first study the wavelength tuning property by applying different bias voltages. Before the experiment, we use DC bias to pre-inject charge into KTN crystal. And the electron injection will be eventually saturated and stabilized. Upon this stage, the charge density eN (i.e., injected electrons) do not change anymore. After the electron injection, we apply different temporary Squared-waves to tune the output wavelength and we think this process has no effect on the charge density eN. Thus the output wavelength is proportional to the applied voltage. The initial wavelength is set at 1550 nm with the bias voltage of about 160 V. With the increase of the voltage applied on the KTN scanner, the wavelength is gradually increased. With a voltage of about 250 V, the output wavelength is measured to be 1600 nm, which correspond to the beam deflection angle of 28.7 ° incident to the grating. As shown in Fig. 2, there is an almost ideal linear relationship between the output wavelength and the applied voltages. The fitting equation, λ(V) = 1456 + 0.58 × V, is in good agreement with Eq. (4). Our experiments also show the consistency of the results with multiple measurements. We measured the long-term stability of our light source for 35 hours at low frequency, which indicating good beam performance with high stability of this wavelength-swept ASE source. The KTN deflector is a key component for fabricating the stable light source of high performance. As shown in Fig. 2, the slight deviation between the measured values and the fitting lines may be due to the KTN lens power fluctuation. The screening effect caused by partially trapped electrons prevents the trapped electrons from reaching an equilibrium state rapidly, which leads to the lens power fluctuation [29,30]. The deviation of the KTN lens power after stabilization is kept within 5% for an operation time of 3,000 hours has been reported [30]. Thus we confirm that a more stable wavelength-swept ASE source can be expected by irradiating with the weak light. Meanwhile, it should be noted that the whole wavelength-swept range is over 80 nm if we continue to change the applied voltage range during the measurements.
Fig. 2 Wavelength tuning as a function of bias voltage. Multiple measurements were taken to verify the repeatability and stability of the light source system.
Next, we study the beam output power, the linewidth, and the noise suppression. As seen in Fig. 3(a) and 3(b), beam output power enhances with the increase of the operation current of the BOA. When the applied current increases to 500 mA, the output beam power can exceed 100 mW (i.e., 20 dBm). We use three prisms to elongate the illuminating area of the collimated laser beam across the grating grooves. Due to this configuration, the output power is much more stable due to the prisms will enhance the spectral resolution and increase the stability of the amplified beam. The fluctuation of output power is less than 2.5 dBm in the wavelength range from 1540 nm to 1610 nm [as shown in Fig. 3(b)]. On the other hand, this beam expansion configuration can greatly stabilize and reduce linewidth. As shown in Fig. 3(d), the linewidths are all less than 0.5 nm in the whole range when different laser amplifier current (from 300 mA to 500 mA) is applied. More importantly, the fluctuation of linewidth is less than 0.1 nm. By contrast, the linewidths are increased with fluctuation of near 0.5 nm when the prisms are removed [as shown in Fig. 3(c)]. Moreover, our light source is capable of delivering high-quality of beam output with relatively low noise. As indicated in Fig. 4, the maximum noise level is about −3.7 dBm and the minimum noise level can reach about −30 dbm. Thus the signal-noise ratio exceeds 24 dB. The above measures show our wavelength-swept ASE source has a good output characterization in the tuning range. Also, when the amplifier current is set to 600 mA, the maximum output power of the beam exceeds 110 mW. There is no report that claims the output power of the light source using KTN deflector reaches over 100 mW before [27,31].
Fig. 3 Characterization of the beam output under different operation current of the BOA. Beam output power without (a) or with three-prism beam expender (b). Linewidth without (c) or with three-prism beam expender (d).
We further study the wavelength-swept speed of our wavelength-swept ASE source by applying high-frequency electric fields. The momentary tuning speed, v, is given by ν = (λm - λm-1) / (tm - tm-1), where λm - λm-1 represents the local wavelength interval, and tm - tm-1 is the time interval between these two measured points. In our experiments, a 220V DC bias is applied to pre-inject charge into KTN crystal and a 100 KHz Sawtooth-wave as additional driving voltage is overlapped with the DC bias. We use DC bias to inject electrons because the electrons can hardly enter the KTN crystal when applying voltage for a short time [29]. The output beam with high-speed wavelength tuning is aligned to a high-resolution transmission grating which acts as a spectral filter to select one wavelength during the dynamic spectral tuning. First, the selected wavelength measured is at 1575.7 nm by using an optical spectrum analyzer, and then the beam is coupled into a high-speed photodetector (Multiplex MTRX192L 10Gbs Receivers). The signal from the photodetector is sent to a fast oscillator to monitor the dynamic transition of the selected wavelength in the tuning process. There are two pulses within one period of the Sawtooth-wave, which is corresponding to the same wavelength of 1575.7 nm since the EO scanner scans the same point twice (back and forth, in one period). Next, we repeat the above steps by selecting another wavelength at 1560.9 nm. As such, we have two similar pulse trains but they slightly shift (referring to the same Sawtooth-wave) since there are corresponding differently wavelengths. From the two measurements, we obtained λm = 1560.9 nm at tm = t0, and λm-1 = 1575.7 nm at tm-1 = t0 + 1.43 μs. Therefore, the momentary tuning speed is v ≈107 nm/s. The two-step measurement is quite accurate since both the wavelength and the time can be precisely determined.
The swept range at high frequency becomes relatively narrow (as indicated in Fig. 5) compared to the case with low frequency. When the peak-peak value of the fast AC voltage is larger than 80 V, the swept range no longer increases. By utilizing either Squared-wave or Sawtooth-wave, we observe similar results of output beam performance. When the KTN scanner is operated at high frequencies, the thermal effect caused by electricity is supposed to decrease its performance, narrowing the swept range [32–36]. Meanwhile, as the EO effect will be saturated in high voltage region [18], thus the swept-range reaches the maximum value in that region.
Fig. 5 Dependence of swept-wavelength range on applied voltage with Squared-wave and Sawtooth-wave electric fields.
4. Conclusions
In conclusion, we have developed an external-cavity wavelength-swept ASE source with high output power and high tuning speed based on the efficient electro-optic effect of beam deflection. The light source exploiting a single BOA is capable of delivering stable output with averaged intensity of 100 mW. The beam linewidth is less than 0.5 nm and the fluctuation of linewidth is less than 0.1 nm even if the current value of the BOA changes from 300 mA to 500 mA. For practical applications, we believe further improvement of some performances can be achieved by efforts through engineering and manufacturing processes, as well as high-quality KTN deflector development. The light source will have not only important applications in scientific research such as biomedical imaging, spectral analysis and sensing, but also potential industrial applications such as optical communications, environmental monitoring, fast spectral inspections for safety and security.
Funding
National Natural Science Foundation of China (NSFC) (Grants No. 61575097 and 11704201); National Natural Science Foundation of Tianjin (17JCQNJC01600).
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