Ultrafast speed, large angle, and high resolution optical beam steering using widely tunable lasers

Gonghai Liu; Qiaoyin Lu; Weihua Guo

doi:10.1364/OSAC.2.001746

1. Introduction

Electronically controlled optical beam steering has attractive features such as rapid response and high resolution when compared with mechanical beam-steering technologies. The approaches for electronically controlled optical beam steering, if they can increase steering speed, provide random access pointing, increase reliability, and reduce costs, they will make a huge impact on a number of applications such as light detection and ranging (LIDAR), free space secure laser communication, real-time three dimensional imaging, driverless cars, etc [1–3]. Various methods have been demonstrated to achieve the electronically controlled optical beam steering, such as acousto-optic scanning, electro-optic deflectors, optical phased array, holographic scanning, liquid crystal, and so on [4].

These methods have their pros and cons. Acousto-optic scanning implements beam deflection based on phonon-elastic effect. The response and steering angle of the acousto-optic scanning is limited by the acoustic wave [5,6]. Meanwhile, the phonon-elastic effect could change the frequency of the input optical wave. This frequency change may disturb the signal carried by the optical wave. Electro-optic deflectors are also effective to achieve electronically controlled optical beam steering with features of rapid response and continuous beam steering because the control voltage and the steering angle can both be continuously changed. However, the working voltage of electro-optic deflectors is generally very high. This characteristic makes the controller difficult and thus limits their application range; also, they are not suitable for large angle beam steering due to the limitation from their aperture size [7,8]. The scanning technology based on optical phased arrays can realize large angle, rapid response and high resolution through electronically adjusting the phase of the various phase shifters within the array [9,10]. The optical phased arrays cannot have perfect performance if the phase shifter element size cannot approach sub-wavelength. The sizes of optical waveguides or fibers are usually much larger than one micron (for example, the typical waveguide size is about 2 µm) [11–16]. Thus, it is extremely difficult to avert grating lobes in the near IR region because the required waveguide separation and size will be less than one micron. Grating lobe is a major limitation of the optical phased array technology [13,14]. It not only severely reduces the light efficiency but also may cause complexity and unreliability in the detected signal. Although the unequally spaced optical phased array technique can reduce grating lobes, the effect is rather limited [13]. Liquid crystal light beam scanning technology uses liquid crystal to realize electronically controlled beam deflection. But due to the fact that the response speed of liquid crystal material is rather limited (10 kHz), liquid crystal light beam scanning technology is mainly used in small angle, high precision beam scanning [17,18].

Although these methods provide more choices to realize electronically controlled optical beam steering, none of them can currently realize large angle, high speed, high resolution and high reliability of optical beam steering. Therefore, their deployment is also limited in applications like driverless cars [3,19]. There exists an eager need for an ultra-fast and high reliability beam-steering device which can provide a large beam steering angle and a small beam divergence at the same time.

In this letter, we report an electronically controlled optical beam steering method based on widely tunable lasers and cascaded transmission gratings, which can realize ultra-fast speed, large angle, high resolution and high reliability at the same time. In this method, wavelength tuning is employed to steer the beam in the direction along the grating (longitudinal direction θ) because the emission angle of the transmission grating is dependent on the incident wavelength. The beam steering speed depends on the speed of wavelength tuning while the beam steering angle depends on the wavelength tuning range and the angular dispersion of the cascaded gratings; the angular resolution depends on the accuracy of wavelength tuning. The transmission gratings have to be cascaded in a particular way, in order not only to increase the angular dispersion but also to ensure a small divergence of the final emitted beam. The swept laser we used is a commercial product with high reliability used in optical communication systems. And the optical beam steering method does not suffer from the side lobe issue.

2. Structure and methods

The beam steering system includes the control unit, the optical circuit unit and the measurement unit, as shown in Fig. 1. The functions of the control unit include the swept laser controlling and data collection. The swept laser we used is a widely tunable sample grating DBR (SG-DBR) laser manufactured by JDSU Corporation. This tunable laser is widely used in optical communications which has a strict requirement on wavelength stability. In the package the tuning sections of the laser have large shunt capacitances connected in order to reduce the noise from current sources. These shunt capacitances limit the tuning speed of the laser to be less than 1 kHz. In order to make the laser tuning more quickly, we removed these capacitances inside the laser package, but added low shunt capacitances on the control circuit board to limit the laser tuning speed to be less than 20 MHz. The SG-DBR laser generates the wavelength tuned light which then passes through a Mach-Zehnder (MZ) modulator to load the desired signal, and then passes through an EDFA to boost the optical power. Before incident onto the first transmission grating of the three cascaded gratings, the light is collimated to have small divergence angle by a collimator. Gold mirrors are used to fold the beam path as shown in Fig. 1. After passing through the gratings, the beam enters into a 4-f lens system to further magnify the beam steering angle. Photodetectors placed some distance away are used to find the emission angle of the beam.

Fig. 1. The structure of beam steering device.

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The commercial SG-DBR laser has a tuning range over 34 nm and side mode suppression ratio (SMSR) over 45 dB within the tuning range. However this level of SMSR usually is obtained statically. In the process of fast wavelength tuning, lots of wavelengths or side modes appear in the transition period of the driving signal which usually takes nearly a stair-case wave. Therefore, the SMSR of the SG-DBR laser severely deteriorates when wavelength is quickly tuned. The intensity and quantity of the side lobes will increase in this case. This will in-turn severely degrades the performance of the beam steering unit. In the paper, we used an intensity modulator after the SG-DBR laser to help relieve this issue. The intensity modulator which has a 10 GHz modulation bandwidth is biased at off-state unless there is a need to generate a short optical pulse or other desired signals. The off-state of the MZ modulator we have used has an additional suppression of its throughput by about 30 dB relative to its on-state. The MZ modulator is turned off in the transition period of the staircase driving signal and turned on in the middle moments of the staircase driving signal where the output wavelength of the SG-DBR laser is stabilized to the best.

We further boosted the optical power by an EDFA after the intensity modulator. The average power can reach 100 mW. Three transmission gratings and two gold mirrors form the sub-unit to generate the fundamental beam steering based on wavelength tuning. Before entering this unit, a very well collimated beam is generated through a two-stage collimator. The beam diameter is about 7 mm, corresponding to a divergence angle round 0.016^° (0.28 mrad) after the collimator. Three transmission gratings (LightSmyth Technology) are concatenated as shown by the optical circuit unit in Fig. 1. The grating has 966.2 lines per millimeter and has a diffraction efficiency over ninety percent.

As is well-known, grating is a standard angular dispersive device used to steer a beam based on changing the incident wavelength. The relationship between the output angle and the incident wavelength is governed by the grating equation as

(1)$$d \times ({\sin {\theta_{\boldsymbol{i}}} + \sin {\theta_{\boldsymbol{m}}}} )= m \times \lambda $$

where d is the grating period, $\lambda $ is the incident wavelength, m is the grating diffraction order, ${\theta _i}$ and ${\theta _m}$ is the incidence angle and the $m$-order diffraction angle, respectively. The grating diffraction order is often the first in order for the highest diffraction efficiency, so there is $m = 1$. According to the grating equation, we can obtain the angular dispersion rate $\psi $ of grating as:

(2)$$\psi = \frac{{d{\theta _1}}}{{d\lambda }} = \frac{1}{{d \times \cos {\theta _1}}}$$

However, if the incident and output angle of the diffraction grating is different, the beam diameter will also be influenced according to

(3)$$\frac{{{\omega _1}}}{{{\omega _i}}} = \frac{{\cos {\theta _1}}}{{\cos {\theta _i}}}$$

where ${\omega _i}$ and ${\omega _1}$ is the diameter of the incident and diffracted beam, respectively; ${\theta _i}$ and ${\theta _1}$ is the angle of incidence and diffraction, respectively; The incident and diffracted beams are Gaussian beam which has a divergence angle related to the beam diameter as:

(4)$$\varphi \cong \frac{\lambda }{{\pi \omega }}$$

where $\omega $ is the diameter of the Gaussian beam, $\lambda $ is the wavelength of the Gaussian beam. So the grating will change the beam divergence angle as [20]:

(5)$$\Gamma = \frac{{{\varphi _1}}}{{{\varphi _i}}} = \frac{{{\omega _i}}}{{{\omega _1}}} = \; \frac{{\cos {\theta _i}}}{{\cos {\theta _1}}}$$

where ${\varphi _i}$ and ${\varphi _1}$ is the divergence angle of the incident and diffracted beam, respectively.

In order to acquire a large steering angle, we expect to maximize the angular dispersion rate. According to Eq. (2), decreasing the value of $\cos {\theta _1}$ is a feasible method because the value of d is limited by the grating. But decreasing the value of $\cos {\theta _1}$ will make $\cos {\theta _i}$ increase, which will in turn increase the divergence angle of the diffracted beam according to Eq. (5). The beam that has a large divergence angle is very difficult to propagate in free space for long distances.

It is desirable to not only improve the angular dispersion rate but also control the change of the beam divergence angle. As explained in the above, it would be difficult to achieve this goal through a single grating. However it is possible through cascading multiple gratings. The angular dispersion rate of the cascaded gratings structure $\psi^{\prime}$ is:

(6)$${\psi ^\prime } = \frac{{d\theta }}{{d\lambda }} \cong \frac{1}{{d \times \cos \theta _1^1}} \times \frac{1}{{d \times \cos \theta _1^2}} \times \cdots \frac{1}{{d \times \cos \theta _1^k}}$$

the change of the beam divergence angle ${\Gamma^{\prime}}$ is:

(7)$$\Gamma ^{\prime} = \frac{{\omega _i^1}}{{\omega _1^1}} \times \frac{{\omega _i^2}}{{\omega _1^2}} \times \cdots \frac{{\omega _i^k}}{{\omega _1^k}} = \frac{{\cos \theta _i^1}}{{\cos \theta _1^1}} \times \frac{{\cos \theta _i^2}}{{\cos \theta _1^2}} \times \cdots \frac{{\cos \theta _i^k}}{{\cos \theta _1^k}}$$

where $\theta _i^k$ and $\theta _1^k$ is the incidence and diffraction angle for the kth grating, respectively; $\omega _i^k$ and $\omega _1^k$ is the diameter of the incident and diffracted beam for the kth grating, respectively.

In order to reduce the change of the beam divergence angle, we need to keep the value of ${\Gamma ^{\prime}}$ close to 1. Under this precondition, adjusting the incidence angle to maximize the angular dispersion rate $\psi ^{\prime}$ according to Eq. (6). Equation (6) and (7) is closely related to the wavelength of the beam. Therefore, it will not be optimal for the whole wavelength tuning range. It is arranged that the center wavelength of the tuning range satisfies the optimum condition. This way the angular dispersion rate can be improved with the smallest change of the divergence angle.

Based on the above understandings, we designed a cascaded gratings structure consisting of three LightSmyth Technologies’ transmission gratings. Optical beam from the collimator enters into the cascaded grating structure with an incident angle of 48.3^°. By numerical calculation, 34 nm wavelength tuning range from 1529 nm to 1563 nm can achieve 9.12^° of beam steering, minimum and maximum emission angle is 45.53^° and 54.65^°, respectively. The beam divergence angle is below 0.019^°, which is slightly worse compared to 0.016^° from the collimator.

In order to further magnify the steering angle, we designed an angular magnification system consisting of one convex lens and one concave lens [20–21]. The sequence is a convex lens first followed by a concave lens as shown in Fig. 2. These two lenses share the same focal plane and the effective focal length of the convex lens is M times that of the concave lens. Then, the steering angle is magnified approximately M times. Unfortunately, the divergence angle is also magnified approximately M times. In our case, according to the lens spec M equals 3.33.

Fig. 2. The structure of an angle amplifier.

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3. Experiments and results

We designed a control unit to quickly tune the SG-DBR tunable laser. The tuning range of the SG-DBR laser is over 34 nm. The laser output spectra are shown in Fig. 3. The SMSR of the laser is over 45 dB within the whole tuning range. We can choose any wavelength within the tuning range through instructions sent from the computer. The wavelength tuning speed is also adjusted by a control program according to the need.

Fig. 3. The SG-DBR tunable laser output spectra ranging from 1529 nm to 1563 nm with 1 nm step.

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A measurement unit was designed to verify the performance of the beam steering device, as shown in Fig. 1. The measurement unit includes of large diameter lens, high-speed photodetectors and 3-axis stages as shown in Fig. 1. Firstly, we adjusted the location of the large diameter convex lens to make its optical axis coincide with the optical axis of the beam steering system. Then, the beam was steered to different angles and was finally focused onto the focal plane of the large diameter lens. The coordinates of the focus point can be found by photodetectors and the 3-axis stages. These coordinates were used to calculate the different steering angle later on. Furthermore through the high speed photodetectors, we can detect the optical signals carried by the steering beam. The measurement unit can characterize the steering angle, steering speed and angular resolution of the beam steering system.

Figure 4 shows the simulation and measurement result of the beam steering angle. We chose eight wavelengths that range from 1529 nm to 1563 nm with a step of 5 nm to measure the steering angle. The laser output spectra for these wavelengths are shown in the inset picture of Fig. 4. Firstly, we measured the steering angle at eight wavelengths without the angle amplifier. We can obtain about 9^° steering angle by 34 nm wavelength tuning, shown by blue star in the Fig. 4. Thus, an efficiency of 0.26^°/nm has been achieved in free space with three cascaded transmission gratings. Then, we measured the steering angle with the angle amplifier. The results are shown by azure triangles in Fig. 4. The range of the steering angle is magnified to 28.28^°, close to the calculate value of 30^°. The error is mainly caused by the lens distortion of the angle amplifier.

Fig. 4. Steering angle in the longitudinal direction versus wavelength without and with the angle amplifier.

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Then we demonstrated the ultrafast one-dimensional beam steering. The SG-DBR laser was tuned at 10 MHz between two wavelengths: 1529 nm and 1563 nm. The spectra of the laser output were captured by an optical spectrum analyzer, as shown in Fig. 5 (a). The spectra demonstrated clearly bimodal behavior at 1529 nm and 1563 nm with the side-mode suppression ratio above 40 dB. The beam with two wavelengths was input onto the optical circuit unit by a single mode fiber, as shown in Fig. 1. The output beam of the optical circuit unit should steer to two directions at 10 MHz steering speed. To check if the beam is really switching between these two directions, two photodetectors are placed onto the corresponding locations on the focal plane of the large diameter lens, to capture the beam. When beam steering is working, two output signals from the two detectors are monitored by two channels of an oscilloscope, simultaneously. The results are shown in Fig. 5(b). Although the duty cycle of the recorded signals has subtle difference, a clear indication of 10 MHz steering speed (>10^8°/s) can be seen. Therefore, fast steering between two angles through tuning the wavelength of the beam has been demonstrated.

Fig. 5. (a) Laser output spectra for two wavelengths and for fast switching between them; (b) Oscilloscope trace from two photodetectors located at positions corresponding to the two steering directions.

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The high angular resolution of beam steering has also been demonstrated. As outlined above, the direction of the output beam depends on the output wavelength of the tunable laser. Therefore we can realize high resolution beam steering by fine tuning the laser output wavelength. The inset picture of Fig. 6 shows the output spectra of the tunable laser ranging from 1544 nm to 1546 nm with a step of 0.2 nm. We measured the beam steering angle versus the output wavelength, as shown in Fig. 6. The result without angular magnification has been shown by orange triangles in Fig. 6, the angular resolution achieved is round 0.05^°. The angular resolution is around 0.15^° when the angle amplifier is working. The angular resolution can be further improved by tuning the SG-DBR laser thermally. Generally, the output wavelength increases by about 0.1 nm in the C band when the laser temperature rises by 1 degree Celsius.

Fig. 6. Laser output spectra ranging from 1544 nm to 1546 nm with a step of 0.2 nm and the beam steering angle in the longitudinal direction versus wavelength.

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4. Conclusion

An ultra-fast, large angle, high resolution and high reliability optical beam steering device based on tunable lasers and cascaded gratings structure has been demonstrated. We realized fast and steady wavelength tuning by controlling the SG-DBR laser tuning signal and the intensity modulator turning-on time cooperatively. 10 MHz tuning speed has been achieved without the side-lobe interference. The cascaded gratings structure can increase the angle dispersion rate, without worsening the beam divergence. The beam steering device has demonstrated 9^° steering angle without angle magnification, with magnification up to 28^° steering angle has been achieved. 10 MHz steering speed (>10^8°/s) has been achieved with high angular resolution of about 0.15^°. The divergence angle of the output beam is about 0.019^° before angle magnification and is increased to about 0.06^° after angle magnification. These characteristics make the beam steering device a good choice for LIDAR, space laser communication and 3D real-time imaging.

Funding

National High-tech R&D Program of China (2015AA017101).

References

1. https://en.wikipedia.org/wiki/Active_electronically_scanned_array.

2. A. Wehr and U. Lohr, “Airborne laser scanning—an introduction and overview,” ISPRS Journal of Photogrammetry & Remote Sensing. 54(2-3), 68–82 (1999). [CrossRef]

3. HDL- 64E in Velodyne Website, http://velodynelidar.com/lidar/hdlproducts/hdl64e.aspx.

4. P. F. McManamon, P. J. Bos, M. J. Escuti, J. Heikenfeld, S. Serati, H. Xie, and E. A. Watson, “A review of phased array steering for narrow-band electrooptical systems,” Proc. IEEE 97(6), 1078–1096 (2009). [CrossRef]

5. W. Akemann, C. Ventalon, J. Léger, B. Mathieu, S. Dieudonné, B. Blochet, S. Gigan, and L. Bourdieu, “Ultra-fast 3D scanning and holographic illumination in non-linear microscopy using acousto-optic deflectors,” Proc. SPIE 10251, 102510L (2017). [CrossRef]

6. J. Mur, B. Kavčič, and I. Poberaj, “Fast and precise Laguerre–Gaussian beam steering with acousto-optic deflectors,” Appl. Opt. 52(26), 6506–6511 (2013). [CrossRef]

7. K. Nakamura, J. Miyazu, M. Sasaura, and K. Fujiura, “Wide-angle, low-voltage electro-optic beam deflection based on space-charge-controlled mode of electrical conduction in KTa1-xNbxO3,” Appl. Phys. Lett. 89(13), 131115 (2006). [CrossRef]

8. S. Hisatake, K. Shibuya, and T. Kobayashi, “Ultrafast traveling-wave electro-optic deflector using domain-engineered LiTaO3 crystal,” Appl. Phys. Lett. 87(8), 081101 (2005). [CrossRef]

9. D. N. Hutchison, J. Sun, J. K. Doylend, R. Kumar, J. Heck, W. Kim, C. T. Phare, A. Feshali, and H. Rong, “High-resolution aliasing-free optical beam steering,” Optica 3(8), 887–890 (2016). [CrossRef]

10. J. Xu, M. Cua, E. H. Zhou, Y. Horie, A. Faraon, and C. Yang, “Wide-angular-range and high-resolution beam steering by a metasurface-coupled phased array,” Opt. Lett. 43(21), 5255–5258 (2018). [CrossRef]

11. S.-C. Wooh and Y. Shi, “Influence of phased array element size on beam steering behavior,” Ultrasonics 36(6), 737–749 (1998). [CrossRef]

12. H. Unz, “Linear arrays with arbitrarily distributed elements,” IRE Trans. Antennas Propag. 8(2), 222–223 (1960). [CrossRef]

13. S. Yin, J. H. Kim, F. Wu, P. Ruffin, and C. Luo, “Ultra-fast speed, low grating lobe optical beam steering using unequally spaced phased array technique,” Opt. Commun. 270(1), 41–46 (2007). [CrossRef]

14. David C. Jones and Chris Stace, “Beam steering of a fiber-bundle laser output using phased array techniques,” Proc. SPIE 5335(1), 125–131 (2004). [CrossRef]

15. W. Guo, P. Binetti, C. Althouse, M. Masanovic, H. Ambrosius, L. Johansson, and L. Coldren, “Two-dimensional optical beam steering with InP-based photonic integrated circuits,” IEEE J. Sel. Top. Quantum Electron. 19(4), 6100212 (2013). [CrossRef]

16. A. Yaacobi, J. Sun, M. Moresco, G. Leake, D. Coolbaugh, and M. R. Watts, “Integrated phased array for wide-angle beam steering,” Opt. Lett. 39(15), 4575–4578 (2014). [CrossRef]

17. J. Lee, D. Kim, Y. Wu, C. Yu, S. Lee, and S. Wu, “High-speed infrared phase modulators using short helical pitch ferroelectric liquid crystals,” Opt. Express 13(20), 7732–7740 (2005). [CrossRef]

18. X. Shang, J. Tan, O. Willekens, J. Smet, P. Joshi, D. Cuypers, E. Islamaj, J. Beeckman, K. Neyts, M. Vervaeke, H. Thienpont, and H. Smet, “Electrically controllable liquid crystal component for efficient light steering,” IEEE Photonics J. 7(2), 1–13 (2015). [CrossRef]

19. J. Zhang and S. Singh, “Visual-lidar odometry and mapping: low-drift, robust, and fast,” in Proceedings of IEEE Conference on Robotics and Automation (IEEE, 2015), pp. 2174–2181.

20. P. Belland and J. P. Crenn, “Changes in the characteristics of a Gaussian beam weakly diffracted by a circular aperture,” Appl. Opt. 21(3), 522–527 (1982). [CrossRef]

21. W. Guo, P. Binetti, C. Althouse, H. Ambrosius, L. Johansson, and L. Coldren, “InP photonic integrated circuit with on-chip monitors for optical beam steering,” in Proceedings of IEEE International Semiconductor Laser Conference (IEEE, 2012), pp. 16–17.

Ultrafast speed, large angle, and high resolution optical beam steering using widely tunable lasers

Abstract

1. Introduction

2. Structure and methods

3. Experiments and results

4. Conclusion

Funding

References

Cited By

Figures (6)

Equations (7)

OSA Continuum