A Mode-Locked Microchip Laser Optical Transmitter for Fiber Radio

Transcription

1 A Mode-Locked Microchip Laser Optical Transmitter for Fiber Radio Amarildo J. C. Vieira +, Peter R. Herczfeld *, Arye Rosen *, Michael Ermold *, Eric Funk, William D. Jemison, and Keith Williams * Center for Microwave-Lightwave Engineering Drexel University 32 nd & Chestnut Street Philadelphia, PA USA Phone: (215) Fax: (215) Dept. of ECE, Lafayette College, Easton, PA USA SFA, Inc., Largo, MD + Motorola Broadband Communication Sector, Horsham, PA SFA, Inc., Largo, MD Naval Research Laboratory, Washington, DC Abstract - This paper is concerned with the optical domain generation and transmission of high-quality millimeter-wave signals for fiber-radio and other applications. The modelocked millimeter-wave optical transmitter described here is based on simple electrooptic microchip laser technology. The transmitter can be designed to operate from a few GHz to 100 GHz and beyond. The residual phase noise of the laser is below 100 dbc/hz at 1 khz off-set which makes it well suited for optically fed millimeter-wave wireless applications. A key feature of the transmitter is its simplicity, the very small number of elements it employs and the high level of integration of the millimeter-wave and photonic components that results in a small, rugged and reliable package. The paper describes the design, fabrication and experimental evaluation of the transmitter. Index Terms- Microchip laser, laser mode-locking, millimeter-wave generation, fiber radio, optical transmitter. 1

2 I. INTRODUCTION Given the increasing demand for high-speed communications, there has been growing interest in developing techniques that can transmit microwave and/or millimeter-waves over optical fiber. There are both commercial and military applications for this type of technology. Commercial applications include personal communication systems (PCS) [1-3], and broadband distribution of interactive multimedia services to the home [4-5]. Examples of military applications include Doppler radar [6] and phased array antenna [7]. The principal advantages of the transmission of high frequency signals over fiber is low attenuation and cost when compared to the conventional coaxial cable or radio transmission, and the large bandwidth even when only part of the available bandwidth is exploited. For example, at 40 GHz, a 10 % bandwidth corresponds to 4 GHz, which is sufficient to allocate 4 times the amount of bandwidth that cable TV service providers can offer. An additional advantage that makes millimeter-wave desirable for fiber radio systems is that these frequencies are highly attenuated by water molecules and oxygen in the atmosphere (i.e. 16 db/km at 60 GHz) [1]. This can be exploited to limit signal propagation within the proximity of the cell, which is essential for wireless secure communication and for frequency reuse. Typical fiber radio distribution system consists of a millimeter-wave optical transmitter at the central station and numerous base stations at the picocell sites. Cost and reliability issues mandate very simple base stations with no local oscillator or sophisticated signal processing [8]. This implies the need for an optical transmitter that can generate all the necessary optical signals that result in the generation of high quality millimeter-waves at a photodetector output located in the base station. Specifically, the detected signal at the base station must include the millimeterwave carrier with suitable modulation for wireless transmission as described in reference [8]. There are alternate methods to achieve this, including direct modulation and modelocking of semiconductor lasers [1, 9], external modulation [10], laser heterodyning [11], and subharmonic optical injection locking [12]. However, these techniques usually lack in performance or are extremely complex and costly. In this paper, a novel optical transmitter based on a mode-locked Neodymium-doped Lithium Niobate (Nd:LiNbO 3 ) microchip laser is described. The mode-locked millimeter-wave optical transmitter (M-MOT) is comprised of two principal parts, the mode-locked microchip laser and an optical modem, as depicted in Figure 1. 2

3 The mode-locked microchip laser has three components: the microchip laser which is housed in a millimeter-wave cavity, the semiconductor diode array that pumps the laser, and a millimeterwave source that locks the phases of the longitudinal laser modes. The output of the mode-locked microchip laser is a train of pulses, the sum of propagating optical modes that can be represented by the following relation: I total = I i e j ( ω φ ) i t i where I i is the intensity of the i-th mode, and ω i and φ i are its optical frequency and phase. The frequency difference between the adjacent modes, ω=ω i -ω i-1 =ω mm is determined by the free spectral range, which in turn is dependent on the effective length of the laser cavity. At the output of the detector the beating of the different modes yields a millimeter-wave signal with the frequency ω mm. The millimeter-wave locking signal assures that the modes are in phase, φ j -φ j- 1=0, in order to produce a very low noise millimeter-wave carrier at the detector. Mode-locked microchip laser Optical Modem Diode pump Millimeter-wave source Microchip laser in a millimeter cavity Information and control signals Mach-Zehnder modulator Transmitter Output Figure 1. Block diagram of the mode-locked millimeter-wave optical transmitter (M-MOT). The optical modem has several functions. The various information signals, namely voice, video, data and others are organized and superimposed on the optical carrier using a Mach- Zehnder or other suitable device. This approach promotes significant agility of frequency and modulation format. Some applications require optical domain filtering which is also carried out 3

4 at this level. In general, the optical modem would also include a detector for the signals originating at the distributed base stations; that is not considered here. The approach taken in the design of the microchip laser is significantly different from past practices in two main aspects: i. The host material for the solid-state microchip laser is an electro-optic crystal, LiNbO 3, which provides for efficient interaction between the optical and electric fields. This is exploited for efficient mode-locking. ii. The laser and the microwave components are fully integrated in a compact, rugged and low cost package, which reduces parasitic effects and therefore provides for better performance. Section II of this paper describes the design and performance of the microchip laser. This discussion includes a general description of the laser as well as the issues involved in optimizing the optical and microwave interaction to achieve efficient mode-locking. Several generations of hardware development are described, each showing a successively higher level of integration. Section III presents both optical domain and millimeter-wave domain measurement results, which is followed by a brief section of conclusions. II. THE MODE-LOCKED MICROCHIP LASER The core of the mode-locked millimeter-wave optical transmitter is the solid-state microchip laser shown in Figure 2. Microchip lasers are monolithic flat-flat optical cavities formed by a short length of gain material with dielectric mirrors deposited directly on their surfaces [13-15]. Since the host material, Lithium Niobate, is electro-optic, the laser chip can be placed in a millimeter-wave cavity to facilitate effective interaction of the optical and millimeterwaves resulting in superior mode-locking performance. Therefore, the motivation for using microchip lasers is driven by three main factors: i. Solid-state lasers are inherently more stable, have lower noise, and in general have a higher output power than semiconductor lasers. ii. The microchip configuration lends itself to a low cost, compact, short length cavity, which is very suitable for mode-locking at high millimeter-wave frequencies. 4

5 iii. Solid-state lasers lend themselves to integration with the microwave subsystems, yielding a low cost, rugged, integrated package that can be manufactured in large quantity. Diode Pump Beam Mode-Locking Signal L c Mode-Locked Output Dielectric Mirrors Doped Electrooptic Crystal Figure 2. Concept of the mode-locked microchip laser. With regard to the design of the mode-locked microchip laser emphasis was on simplicity and minimizing the number of component employed. The following specific goals were established: i. Efficient optical coupling of the semiconductor diode pump beam into the microchip laser ii. Efficient coupling of the laser output into a single-mode fiber, iii. Optimal interaction between the millimeter-wave and optical fields iv. Providing a simple, low noise and low cost millimeter-wave input signal for the mode locking, and further reduction of the phase noise as required for some applications. IIa. Microchip Laser Crystal Design The microchip laser employs a y-cut Lithium Niobate (LiNbO 3 ) crystal as a host material, doped with 1-atm % of Neodymium (Nd) to provide for optical gain. In addition, 5-atm % of Magnesium Oxide (MgO) was also added to the melt before pulling to reduce the effects of photorefractive damage [16]. Lithium Niobate was selected for the laser because of its excellent optical and electro-optical properties, principally its large electro-optic coefficients. Dielectric mirrors were directly deposited on the crystal surfaces forming an optical cavity. The length of the laser cavity was designed to be 3.48 mm, which corresponds to a roundtrip time of 50 ps, or 5

6 an axial mode spacing of 20 GHz, matching the desired millimeter-wave subcarrier for this particular experiment. IIb. Millimeter-wave Cavity Design In order to achieve low-noise millimeter-wave generation, the microchip laser must be mode-locked to an external millimeter-wave signal. In our work, this millimeter-wave modelocking signal is applied to the Nd:LiNbO 3 microchip laser through a reentrant microwave cavity as indicated in Figure 3. Coarse Tuning Center Cylinder Cavity Tuning Cylinder Pump Beam Output Beam Microchip laser in cavity gap Figure 3. Reentrant microwave cavity. The reentrant coaxial cavity was designed to be resonant at 20 GHz. The basic design used a transmission line analysis where the cavity is modeled as a shorted length of loss-less coaxial transmission line that is terminated by the capacitance of the gap between the center conductor and the bottom of the cavity [17]. The cavity length is selected such that the input impedance of the shorted transmission line provides an inductance that cancels the gap capacitance at the desired resonant frequency of 20 GHz. Both the fringing capacitance of the gap [18] and the capacitance of the solid-state laser chip are included in this analysis. Thus, a first order approximation of the required cavity length is obtained. The mechanical design allows both course and fine frequency tuning by providing cavity length adjustment about the nominal value and by a tuning screw in the side wall of the cavity as shown in Figure 3. In addition, the mechanical design also incorporates an alignment groove in 6

7 the bottom of the cavity to facilitate the alignment of the laser chip and input and output optics. The 20 GHz signal is coupled into the cavity by a small loop probe located on the cavity sidewall. In addition, a temperature sensor and a thermoelectric cooler were incorporated into the microwave cavity in order to keep the cavity and the laser at a fixed temperature. IIc. Microchip Laser Mode Locking In order to facilitate efficient mode-locking, it is desired to optimize the interaction between the optical and millimeter-wave fields within the solid state laser crystal. Since the single Nd:LiNbO 3 laser chip provides both the gain medium for lasing and electro-optic properties for mode-locking, the laser chip is placed in the center of the gap between the center conductor and the bottom of the cavity where the electric field is highest. This location will provide the strongest electro-optic interaction and should provide the best mode-locking performance. However, it should also be noted that mode-locking theory is not developed for this case where both the gain (for lasing) and electro-optic (for mode-locking) properties are in the same medium and occupy the entire laser cavity. Therefore, there is some uncertainty with respect to the field distribution required within the crystal for optimum mode-locking. In order to gain insight into the mode-locking, a finite element computer simulation of the laser and cavity was initiated using a commercially available High Frequency Structure Simulator (HFSS). The reentrant cavity, solid state laser chip, alignment groove, coupling probe, and tuning screw are all incorporated into the model. The simulated resonance frequencies for this model are accurate to within 1% of the measured values determined via an S 11 measurement as shown in Figure 4. The electric field distributions in the reentrant cavity and laser chip were also computed for the dominant mode at GHz. The z-component of the electric field for this mode corresponding to an x-y plane cut through the center of the laser chip is shown in Figure 5. The field distribution is stronger in the laser chip than in the open cavity as desired. This is expected since the dielectric constant of the laser chip is high (26 in the z-direction). The field concentration is symmetric along the longitudinal axis of the laser chip with the highest field concentration occurring at the center of the laser. It is expected that the insight gained from these and ongoing simulations will ultimately lead to a complete mode locking theory for this unique laser. 7

8 Figure 4. Measured S11 of the reentrant cavity with the microchip laser. Dotted lines indicate simulated resonant frequencies. Tuning Screw Center Cylinder Microchip laser Microchip laser in cavity gap Loop Antenna Cavity inside wall (a) (b) Figure 5. (a) Simulated z-component of electric field inside the microwave cavity. X-Y plane is located at the center of the laser chip. (b) Simulated z-component of the electric field inside the microchip laser. X-Y plane is located at the center of the chip IId. System integration During the development of the M-MOT transmitter the issues outlined earlier were systematically addressed, leading to three different mode-locked microchip laser packaging 8

9 configurations. Each generation used the same laser chip and basic reentrant cavity design. However, each successive configuration employed a higher level of integration. The three generations of laser hardware development are discussed below. First Generation baseline design: The main issue addressed in the first transmitter generation was the optical alignment. The alignment of the pump beam and the collimation of the laser output are critical to the transmitter performance. The pump for this generation was an external semiconductor pump laser diode operating at 814 nm with an output power of 280 mw. The pump beam is coupled to the microchip laser through a fiber imbedded in a ceramic sleeve. The ceramic sleeve is aligned with the collimator and the laser placed in between them into an alignment grove designed to aid in the mechanical alignment of the semiconductor laser chip. The picture for this packaging is shown in Figure 6. The microchip laser output was coupled to a single-mode fiber using a fiber collimator. The mode-locking signal was obtained from an external frequency synthesizer or a Gunn oscillator. Coarse Tuning Input fiber Output fiber Fine Tuning Figure 6. Packaged unit with an external pumping scheme. Second Generation integrated mm-wave source: Millimeter-wave generation with good stability and noise performance was obtained using the baseline configuration previously described. However, while external synthesizers provide good millimeter-wave performance, they are bulky and costly. Therefore, the transmitter second generation emphasized the replacement of the synthesizer with compact, low cost millimeter-wave signal sources. One of the options that were investigated involves the use of an external Gunn oscillator. Its noise and power levels are compatible with the requirements of the M-MOT. Moreover, since the Gunn 9

10 oscillator and the reentrant microwave cavity have basically the same structure, they can both be mounted together, leading to a more compact configuration. The RF power injected for mode locking is +10dBm. The optical transmitter, which is comprised of these two cavities, forms a small, rugged package. The spectra of the free-running and mode-locked signals are depicted in Figure 7. The phase noise is adequate for many communications applications, but can be improved by using a lower phase-noise source such as a dielectric resonator oscillator. Efforts are currently underway to integrate a low noise Gunn oscillator into the microchip laser housing that will result in further size reduction. (a) (b) Figure 7. Microwave spectrum of the laser output for (a) free-running and (b) mode-locked microchip laser. Third Generation integrated pump: The third generation of the transmitter is already designed and under test. It is a more compact configuration with an internal semiconductor pump. A laser diode chip is incorporated into the reentrant microwave cavity as shown in Figure 8. With this approach only one temperature controller is used to stabilize the pump and reentrant cavity. Once again, the pump beam and the collimator are aligned, and the laser chip is adjacent to the pump. This configuration is easier to work with and allows for systematic repeatability. These efforts in system integration resulted in an integrated package housed in a rugged cavity that is comprised of the laser chip, two active components, namely the diode pump and the Gunn diode each driven by single DC inputs, one temperature controller, and one optical output. 10

11 Laser Diode Chip Microchip Laser Alignment Grove (b) Figure 8. (a) Shows the packaged unit with millimeter-wave input and optical output. (b) Depicts the inside of the cavity, with the top removed. The pump diodes are permanently affixed and the laser chip is inserted into the grove. III. EXPERIMENTAL CHARACTERIZATION OF THE MODE-LOCKED MICROCHIP LASER A thorough characterization of the mode-locked optical transmitter requires measurements in both the optical and millimeter-wave domains. This section describes the tests performed in both of these domains. The 20GHz mode-locking signal from a Gunn oscillator or a synthesizer was applied to the reentrant microwave cavity. The microwave frequency was set to correspond to the free spectral range of the laser and the microwave cavity was adjusted to resonate at this particular frequency. The mode-locked laser output was applied to an optical spectral analyzer, microwave spectrum analyzer and high frequency sampling oscilloscope for the optical, microwave, and time domain measurements, respectively. IIIa. Optical and Time Domain Characterization The optical spectrum of the mode-locked laser was measured using an optical spectrum analyzer with a resolution of 0.05nm (i.e. 12.8GHz) at 1.084µm. The result, shown in Figure 9, reveals stable modes 20GHz apart that displays a very stable and well-defined structure, with a Lorentzian-like distribution. For the time domain measurement, the mode-locked longitudinal modes, detected by a high speed InGaAs Schottky photodiode (New Focus model 1014), was fed into a high frequency sampling oscilloscope (Tektronix model CSA 803). Figure 10 depicts the waveform 11

12 obtained when a 12.6dBm mode-locking signal was applied to the reentrant microwave cavity. The waveform corresponds to a train of optical pulses with 18.6ps width and 20GHz repetition rate. The shape of these pulses is limited (broadened) by the bandwidth of the measuring instrumentation due to attenuation of higher order harmonics. 0.3 Power (mw) Wavelength (nm) Figure 9. Optical spectrum of the mode-locked laser output. The resolution is 0.05nm. Figure 10. Time domain measurement of the mode-locked laser output. The upper trace is the driving signal and the lower trace corresponds to the laser pulse. The pulsewidth measured was 18.6 ps. 12

13 IIIb. Millimeter-Wave Domain Characterization The corresponding microwave domain signal, after the mode-locked modes are beaten in a high-speed photodetector, is shown in Figure 11. As in the previous measurement, a very stable signal with peak output at 20GHz was observed. The 23dBc side-bands at 350 khz off-set frequency are due to the laser relaxation oscillation. We intend to suppress these by employing feedback control to the pump laser. Peak suppression of better than 30dB has been reported in the literature [19] Power (dbm) Frequency (GHz) Figure 11. Microwave spectrum of the mode-locked laser output. The frequency span and the resolution bandwidth for this measurement is 1MHz and 10KHz, respectively. The phase and the amplitude noise of the generated millimeter-wave were also measured. The single-sideband phase-noise measurement was performed using HP3048A measurement system. The result, shown in Figure 12, indicates that the absolute phase noise is determined by the modulating source, the Gunn oscillator in this case. The residual phase noise was measured as well and found to be negligible over the relevant bandwidth for most applications (< -110 dbc/hz at 10 KHz). The absolute phase noise integrated from 10 khz to 20 MHz is 38 dbc. The amplitude noise of the millimeter-wave generated using the microchip laser is shown in Figure 13. Discrete spurious signals are plotted separated from the noise power density. The - 35 dbc discrete peak at 460 khz is due to the laser relaxation oscillation. The exact frequency of this oscillation is dependent upon our alignment of the output coupler. The integrated amplitude noise (discrete spurious interference not included) from 10 KHz to 20 MHz is 27 dbc. 13

14 Figure 12. Absolute single-sideband phase noise measurement at 20 GHz. The dotted line is the Gunn oscillator phase noise, the darker line is after the M-MOT and the lighter and lower line is the residual phase noise measurement. Figure 13. RF Amplitude noise measurement (in dbc/hz) as a function of the carrier offset frequency for the mode-locked laser at 20 GHz. The vertical lines indicate discrete spurs. 14

15 IV. SUMMARY AND CONCLUSIONS A novel millimeter-wave optical transmitter was designed, fabricated and experimentally characterized. The heart of the transmitter is a mode-locked electrooptic microchip laser whose most desirable attributes are simplicity, compactness, high level integration of the millimeterwave and photonic components and superior performance, particularly regarding phase noise. The transmitter was conceived as part of a fiber-radio system and it meets all the requirements for this application. Table I summarizes the important characteristics of the microchip laser subsystem. Table I Summary of the microchip laser subsystem results Parameter Value Microchip laser host material LiNbO 3 Microchip laser doping Nd (1 atm %) Microchip laser size (for 20 GHz signal generation) 1x1x3.4 mm Lasing wavelength µm Optical linewidth <25KHz Lasing efficiency 18% Laser output power Millimeter-wave frequency Millimeter-wave mode locking power (threshold) 50 mw 20 GHz 6 dbm Modulation index 98% linewidth of millimeter-wave signal Phase noise of millimeter-wave signal - integrated Amplitude noise of millimeter-wave signal - integrated <80 KHz -38dBc -27dBc 15

16 Current efforts, with promising preliminary results, are focusing on developing a chip laser in Er:LiNbO 3. The Erbium doped laser will operate at the more conventional wavelength of µm. A more powerful pump source and better optical coupling from the pump into the laser is expected to increase the optical output power to several hundred milliwatts. The millimeter-wave frequency generated by the laser can be readily extended to 100 GHz and beyond by proper scaling of the laser chip. Subharmonic mode-locking would also allow a lower frequency locking source to be used. Specifically, we successfully obtained a stable 40 GHz mode-locked component at the laser output when locked to either a 20 GHz or to a 40 GHz source. The simplicity of the mode-locked microchip laser coupled with efficient, integrated packaging of the millimeter wave and photonics components imply high reliability and reduced cost. Although the Mach-Zehnder interferometer is not discussed here, it should be pointed out that since both the microchip laser and the modulator use LiNbO 3 there is a distinct possibility for their chip level integration. This is presently studied, as it would lead to simplified optical coupling between these devices that translates to better performance and potentially greatly reduced cost. The generation and transmission of high quality millimeter-wave signals has numerous applications. The M-MOT device discussed in this paper was specifically designed for fiberradio and LMDS, and it meets and exceeds the requirements for this particular application. However by suitable tailoring of the device it can be adopted to other applications such as the optically fed and controlled millimeter wave phased array antenna and high sampling rate photonic Analog-Digital converter (ADC). Some other applications, like Pulsed Doppler radar at 94 GHz, have more stringent requirements particularly in terms of phase noise (-130 dbc/hz or less at 100 KHz off-set). Optical domain feed back can dramatically increase the Q of the millimeter wave resonator and thus reduce noise [20]. Conventional optical feedback employs ~2 km fiber (a compact spool), a high-speed detector, millimeter-wave amplifier and filter, which add noise and cost. The two terminal Gunn device can be directly illuminated by the light and eliminating the need for the detector, amplifier and filter. 16

17 ACKNOWLEDGMENTS We would like to thank Greg Mizell and Greg Quarles from VLOC for growing and processing the Nd-doped Lithium Niobate crystals and Ron Esman and Paul Matthews for valuable suggestions. This work was support by the Office of Naval Research (ONR) under the grant # N REFERENCES [1] H. Ogawa, D. Polifko, S. Banba, Millimeter-Wave Fiber Optics Systems for Personal Radio Communication, IEEE Trans. Microwave Theory Tech., Vol. 40, No. 12, pp , December, 1992 [2] H. Schmuck and R. Heidemann, High Capacity Hybrid Fibre-Radio Field Experiments at 60GHz, International Topical Meeting on Microwave Photonics, pp , Kyoto, Japan, [3] S. Komaki et al., Proposal of Fiber and Radio Extension Link for Future Personal Communication, Microwave and Optical Technol. Lett., Vol. 6, No. 1, pp , January, [4] T. E. Darcie et al., Wide-Band Lightwave Distribution System Using Subcarrier Multiplexing, IEEE J. Quantum Electron., Vol. QE-7, No. 6, pp , June, [5] Z. Ahmed et al., Millimeter-wave (37GHz) Transmission Data (up to 500Mb/s) in an Optically Fed Wireless Link Incorporating a Hybrid Mode-Locked Monolithic DBR Laser, International Topical Meeting on Microwave Photonics, pp , Kyoto, Japan, [6] E.C. Niehenke.and P. Herczfeld, An optical link for W-band transmit/receive applications, Microwave Symposium Digest, 1997 IEEE MTT-S International, Vol. 1, p. 35, June [7] A. S. Daryoush et al., Optically controlled phased array at C-band, APS/URSI International Symposium, Vol.1. p. 466, July [8] "Fiber Radio Using a Mode-Locked Microchip Optical Transmitter - System Considerations and Performance Evaluation", W. D. Jemison, P. R. Herczfeld, E. Funk, K. Williams, A. Paolella, A. Vieira, J. SooHoo, W. Rosen, J. Adams, A. Rosen, A. Joshi, D. Novak, R. B. Waterhouse, submitted to the special issue of the IEEE Transactions on MTT, October [9] D. Novak, C. Lim, H. F. Liu, Optimization of Millimeter-Wave Signal Generation Using Multi-Electrode Semiconductor Lasers with Subharmonic Electrical Injection, International Topical Meeting on Microwave Photonics, pp , Kyoto, Japan, [10] K. Noguchi, O. Mitomi, H. Miyazawa, and S. Seki, A Broadband Ti:LiNbO3 Optical Modulator with Ridge Structure, IEEE J. Lightwave Technol., Vol. LT-13, No. 6, pp , June, [11] G. J. Simonis and K. G. Purchse, Optical Generation, Distribution, and Control of Microwaves Using Laser Heterodyne, IEEE Trans. Microwave Theory Tech., Vol. 38, No. 5, pp , May, [12] A. S. Daryoush, Optical Synchronization of Millimeter-Wave Oscillator for Distributed Architectures, IEEE Trans. Microwave Theory Tech., Vol. 38, No. 5, pp , May,

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