Microwave Signal Generation Based on Optical Heterodyne and its Application in Optical Telecommunication System

A. Baylón-Fuentes1, P. Hernández-Nava1, I. E. Zaldívar-Huerta1, J. Rodríguez-Asomoza2, A.

García-Juárez3, G. Aguayo-Rodríguez1

1Instituto Nacional de Astrofísica Óptica y Electrónica, Apdo. Postal 51 y 216. Puebla, C. P.

72000, México 2Universidad de las Américas-Puebla, Depto. de Ing. Electrónica, Ex-hacienda Sta. Catarina

Mártir, Cholula, Puebla, C. P. 72820, México 3Universidad de Sonora, Depto. de Investigación en Física. Blvd. Luis Encinas y Rosales S/N,

Hermosillo, Sonora, C. P. 83000, México abaylon@inaoep.mx, pherna@inaoep.mx, zaldivar@inaoep.mx, jorge.rodriguez@udlap.mx,

agarcia@cajeme.cifus.uson.mx, gar@inaoep.mx

Abstract Microwave signal generation based on optical heterodyne technique is presented. In order to demonstrate the feasibility of this technique, it is important to have an application on optical telecommunications systems, where the microwave signal obtained is used to drive a Mach-Zehnder Intensity modulator as part of an external modulation scheme. At the same time the microwave signal is used to synchronize the reception stage. Finally, a practical application is shown when TV-signal is transmitted through 28 Km of standard single mode fiber. 1. Introduction

Optical generation of microwave signals has many potential applications such as broad-band wireless, radar, software-defined radio, and satellite communications [1]. Among the most common techniques of generating microwave signals through optical techniques are: Optical Heterodyne Technique [2], Optical Injection Locking [3], Optical Phase-Locked Loop (OPLL) [4], by using a Mach–Zehnder Modulator (MZM) in combination with Fiber Bragg Gratings (FBGs) [5], and, by using some Erbium-Doped Fiber Amplifiers (EDFAs), a MZM, and FBGs [6]. However, the techniques reported in references [3], [4], [5], and [6] are complicated and expensive due to the use of several electrical and electro-optical devices. In this sense, optical heterodyne technique has a relatively simple configuration, easy operation, relatively low cost, and possibility of continuous tuning of the beat frequency over a wide range [2]. This technique is capable of

generating frequencies potentially into the terahertz (THz) band, limited only by the bandwidth of the photo-detector used. Considering the advantages of this last technique, in this paper we propose to use the optical heterodyne technique for microwave signal generation in the frequency range of 0.01-5.0 GHz. The main contribution of this work resides on demonstrate the use of the microwave signals on the field of optical telecommunications. For this goal, we demonstrate the use of a particular microwave signal (2.8 GHz), to drive a Mach-Zehnder Intensity modulator that form part of an external modulation scheme. In particular, an analog TV-signal is transmitted through 28-Km of standard single-mode fiber using the microwave signal as electrical carrier as well as demodulated signal. After this introduction, this paper is organized as follows: In Section 2, the operation principle of optical heterodyne technique is introduced. Section 3 is devoted to show the experimental results. Finally, conclusions are given in Section 4. 2. Operation principle.

The common approach to generate a microwave signal based on optical heterodyne technique is to use two lasers. One laser is wavelength fixed, and the other is wavelength-tunable [2]. Optical heterodyne technique is based on a phenomenon called optical beating and occurs when two different optical wavelengths are mixed and detected using a fast PD. The resulting signal at the output of the PD would then have a frequency equal to the spectral spacing between the two wavelengths. The

addition of two optical sine waves with somewhat different frequencies is in fact an AM modulated optical carrier, where the envelope of the composite signal is the desired microwave difference frequency. Thus, the function of the PD is to detect the envelope of the optical carrier, which is the microwave signal. The difference in frequency between the two optical sources can be easily tuned to any desired frequency up to the GHz range. Thus, the main frequency limitation of this approach is the PD bandwidth [1].

Figure 1. Optical heterodyne technique for generating microwave signals

Fig. 1 illustrates an optical heterodyne scheme, where the electric fields with constant amplitude at the output of each optical source can be expressed by [2]

( )[ ]( )[ ])(2exp)(

)(2exp)(

2222

1111

ttjEtEttjEtE

θπνθπν

+=+= (1)

where 1E and 2E are the amplitude, 1ν and 2ν are the corresponding optical frequencies, and )(1 tθ , )(2 tθ are the relative random phases due to the phase noise of each laser. Assuming that both lasers have the same polarization direction, the electric field at the output of the optical coupler can be then expressed as:

2121 )()()()( EEtEtEtEtE SS +=⇒+= (2)

and the corresponding optical power is defined by:

)()( * tEtEP sss ⋅= (3)

Elaborating upon (3), the resulting optical power is the following:

[ ])()()(2coscos2

2121

2122

21

tttvvEEEEPs

θθπφ

−+−⋅++= (4)

where |ν1-ν2|=Δν is the frequency of the RF signal at the PD output. θ1(t)-θ2(t)=θ(t) is the random phase of the RF generated signal, which depends on the two laser phase noise. φ is the relative polarization difference between the two sources and should be minimized to obtain maximum

power [7]. As a result, the electric fields of the electromagnetic waves resulting from laser sources have the same state of polarization. The corresponding photo-current generated by the PD is obtained as:

[ ])(2coscos

2)(

2121

21

vvPP

SPPSSPI s

−

⋅+−==

πφ (5)

where S is the photodiode sensitivity in A/W. Eq. (5) means that the signal achieved by optical heterodyne is composed of a continuous wave component and another component at the beat frequency Δν, which is the frequency difference between the lasers, and is determined as:

21

21

21 λλλλ

λλλν

cc −=Δ=Δ (6)

here 1λ and 2λ are the central wavelength for each laser, and Δλ is the wavelength difference between the optical sources. The laser optical frequency is very dependent on the bias current and the temperature of the junction. To minimize frequency fluctuations, it is essential to perfectly control these two parameters. 3. Experimental setup and results.

Based on above analysis, an experiment using optical heterodyne to generate microwave signals in the range of 0.01 to 5.0 GHz is performed. First, we describe the experiment. Next, we evaluate the performance of the external modulation scheme to transmit analog TV-Signal using the microwave signal as an electrical carrier. 3.1. Tunability of the microwave signals obtained in

the photodiode The schematic diagram of our experimental set up is

shown in Fig. 2. We have used two DFB lasers. Laser 1 was a fiber coupled DFB (Thorlabs, model S3FC1550) with a central wavelength of 1550 nm. This optical source is tuned by temperature. Laser 2 was a tunable laser (New Focus, model TLB-3902) which was tuned over the C band (1527.22 to 1563.45 nm) with channel spacing of 25 GHz.

Figure 2. Schematic diagram of the experimental setup used for microwave signal generation

Polarizationcontroller

Coupler50:50 Photo

detector

Laser 1

Laser 2

Polarizationcontroller

ν1

ν2

I→

SEEs

The outputs of both optical sources were coupled to optical isolators (OI) in order to avoid reflection to the optical source and consequently instabilities. A pair of polarization controllers (PC) allowed matching the polarization of the light issued from each optical source. The two optical beams were combined by means of a 3-dB optical coupler. One output port of the coupler was launched to a fast PD (MITEQ, model 125G-A, typical gain of 13 dB, and a 12.5-GHz bandwidth at -3 dB). A beat signal was obtained at the output of the PD, which is displayed by an Electrical Spectrum Analyzer (ESA), (Agilent model E4407B). The second output port of the optical coupler was connected to an Optical Spectrum Analyzer (OSA), (Anritsu model MS9710C) in order to observe both optical spectrums, (Fig. 2). To obtain a microwave signal, in a first step, we biased the tunable laser and displayed its optical spectrum on the OSA screen. In a second step, the DFB laser was also biased, fixing an optical power of 2.2 mW and its central wavelength was settled as near as possible to the central wavelength of the tunable laser.

1550.0 1550.1 1550.2 1550.3 1550.4 1550.5 1550.6 1550.7

-40

-30

-20

-10

0

10

20

Laser TLB-3902 Laser S3FC1550

λ2 =1550.3435 nm

λ1 =1550.3197 nm

Pow

er (d

Bm

)

Wavelength (nm)

Δλ=0.023739 nm

Figure 3. Optical spectrum corresponding to the mixed optical sources. The peaks located at λ1=1550.3197 nm and

λ2=1550.3435 nm corresponds to the tunable and DFB lasers, respectively.

As can be seen from Fig. 3, the value of Δλ=0.023739

nm corresponds to the wavelength difference between both lasers and it corresponds to the beat signal frequency of 2.8 GHz. A precise control of the difference, between the two central wavelengths and by consequence over the frequency difference, was obtained by tuning the DFB. The wavelength variation of the laser source was obtained by changing the junction temperature between 22.8 ºC, 23.2 ºC and 23.7 ºC corresponding to the frequency range of 0 to 5.0 GHz. Fig. 4 illustrates the electrical spectrums measured for generated microwave signals located at f1=1.0, f2=2.0, f3=2.8 and f4=4.0 GHz respectively. We can appreciate that the microwave signals are in good

agreement with theoretical value given by Eq. (6). In particular, the frequency of the microwave drive signal is set at 2.8 GHz.

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0-40

-30

-20

-10

0

10

20

Pow

er (d

Bm

)

Frequency (GHz)

1 GHz 2 GHz 2.8 GHz 4 GHz

f1 f2 f3 f4

Figure 4. Spectrum for the microwave signal generated by optical heterodyne technique using the arrangement illustrated

in Fig. 2. 3.2. Evaluation of the external modulation scheme.

To show potential applications in the field of telecommunications, a transmission system point to point under the scheme of external modulation was carried out using the microwave signal previously obtained. Fig. 5 illustrates the experimental setup that we used for transmitting and recovering analog TV-signal.

Figure 5. Experimental setup for transmitting and recovering analog TV-signal.

The electrical signal issued from the PD1 was divided

by using a 3-dB power divider (Mini-circuits, model ZFSC-2-10G). One of their ports is launched to an electrical frequency mixer (Mini-Circuits, model ZMX-8GLH, 3700 to 8000MHz, low conversion loss 5.5 dB), and mixed with an analog NTSC TV-signal of 67.5 MHz (TV channel 4). The result of the combination of these

electrical signals (67.5 MHz + 2.8 GHz) is illustrated in Fig. 6.

2.55 2.60 2.65 2.70 2.75 2.80 2.85 2.90 2.95 3.00 3.05-5

-4

-3

-2

-1

0

1

Am

plitu

de (u

. a.

)

Frequency (GHz)

Modulated Signal

Figure 6. Electrical spectrum of the modulated signal using the

arrangement illustrated in Fig. 5

By other side, the light issued from a DFB3 laser emitting at 1313 nm (model WX-8304BE-CC) was injected to a 20-GHz bandwidth Mach-Zehnder intensity modulator (JDS-APE, model AM-150). A polarization controller is used to adjust the polarization state of the light into the intensity modulator. The RF signal issued from the electrical mixer was applied to the electrodes of the modulator. The modulated light was guided into a coil of 28.2466-Km of single-mode standard fiber (SM-SF). The real length of the coil of the SM-SF was measured by means of an OTDR. The optical signal after transmission was recovered by another fast PD2 (MITEQ model DR-125G-A) and amplified. After that, another electrical mixer allows to suppress the microwave signal that played the role of modulated signal.

56 58 60 62 64 66 68 70 72 74 76 78-3.5

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

Am

plitu

de (u

. a.

)

Frequency (MHZ)

TV signal Recovered

Signal

Figure 7. Recovered signal vs original signal

Finally, the recovered analog TV-signal is amplified and connected to the ESA in order to measure the signal quality (Fig. 7). The recovered video was displayed on a standard TV monitor. 4. Conclusions

In summary, we have proposed and experimentally demonstrated the use of optical heterodyne technique to generate microwave signals. High-quality and stable microwave signals in the frequency range of 0 to 5 GHz were generated. Because frequency of generated signals depends on the wavelength difference of two lasers, temperature control of the lasers was monitored in order to ensure wavelength stability and frequency stability of the microwave signals. The microwave signal was used to drive a Mach-Zehnder Intensity modulator as part of an external modulation scheme. TV-signal was transmitted through a long distance optical link and recovered without degradation. Finally, we can add that microwave signals generated by this technique have tremendous application to drive microwave photonic filters and photonic devices. 5. Acknowledgments

A. Baylón-Fuentes, P. Hernández-Nava, and G. Aguayo-Rodríguez would like to thank to the Mexican Consejo Nacional de Ciencia y Tecnología (CONACyT), for the scholarships number 235279, 235293, and 98155, respectively. 6. References [1] Asher Madjar and Tibor Berceli, “Microwave Generation

by Optical Techniques-A Review”, Proc. of the 36th European Microwave Conference, pp. 1099-1101, September 2006

[2] P. Dherbecourt, O. Latry, E. Joubert, P. Dibin, M. Ketata, “Achieving of an optical very high frequency modulated wave source using heterodyne technique”, Optics Communication, 202 (2002), 81-90

[3] Zhichao Deng and Jianping Yao, “Photonic Generation of Microwave Signal Using a Rational Harmonic Mode-Locked Fiber Ring Laser”, IEEE Trans. Microw. Theory Tech., vol. 54, no. 2, Feb. 2006

[4] H. Rideout, J. Seregelyi, S. Paquet, and J. P. Yao, “Discriminator-Aided Optical Phase-Lock Loop Incorporating a Frequency Down-Conversion Module”, IEEE Photon. Technol. Lett. vol. 18, no. 22, pp. 2344-2346, Nov. 2006

[5] Guohua Qi, Jianping Yao, Joe Seregelyi, Stéphane Paquet and Calude Bélisle, “Generation and Distribution of a Wide-Band Continuously Tunable Millimeter-Wave Signal With an Optical External Modulation Technique ”, IEEE Trans. Microw. Theory Tech. vol. 53, no. 10, pp 3090-3097, Oct. 2005

[6] Wiberg A. Pérez-Millán, P. Andrés M. V. and Hedekvist P. O. “Microwave-Photonic Frequency Multiplication

Utilizing Optical Four-Wave Mixing and Fiber Bragg Gratings”, J. Lightw. Technol., vol. 24, no. 1, pp. 329-334, Jan. 2006

[7] P. Dherbecourt, O. Latry, E. Joubert, M. Keteta, “Generation of an amplitude modulated optical wave up to 275 GHz by laser wave mixing for optical component bandwidth measurement”, Optical Engineering Lett. 41, (7), pp 1469-1470, July 2002