Procedia Technology 11 ( 2013 ) 950 – 954

2212-0173 © 2013 The Authors. Published by Elsevier Ltd.Selection and peer-review under responsibility of the Faculty of Information Science & Technology, Universiti Kebangsaan Malaysia.doi: 10.1016/j.protcy.2013.12.280

The 4th International Conference on Electrical Engineering and Informatics (ICEEI 2013)

Preliminary Study for Microwave Generator:Experimental Characterization of Stability of

Distributed Feedback-based Laser Diode

Iyon Titok Sugiartoa,b, Bambang Widiyatmokoa, Achmad Munirb,*aResearch Center for Physics, Indonesia Institute of Sciences

Kompleks PUSPIPTEK Serpong Tangerang Selatan, Banten, IndonesiabRadio Telecommunication and Microwave Laboratory

School of Electrical Engineering and Informatics, Institut Teknologi BandungBandung 40132, West Java, Indonesia

Abstract

Oscillator is an electronic circuit that is designed as a signal generator in which its frequency can be changed. To obtain a signal in high-frequency, normally it is carried out by multiplying the beat signal. However, in the dielectric resonator system, multiplication of the signal awakens the noise to be double as folding of beat frequency. Therefore to avoid the noise, in this paper the oscillator as microwave generator is proposed to be generated by combining two laser diodes with optical heterodyne techniques. In the preliminary study, the experimental characterization of stability of laser diode based on distributed feedback (DFB) Eudyna FLD5F15CX-H9310 is reported to connection to obtain a stable laser wavelength. The results indicate a variationin the legible wavelength from 1551 – 1553nm for the temperature change of 15 – 30oC. The working principle of the system is to vary the temperature of peltier element from diode laser which affects the wavelength of laser. While from the characterizationresult of injection current, it shows that the threshold current (Ith) of laser diode for temperature condition of 18oC is 14.5mA.

© 2013 The Authors. Published by Elsevier B.V.Selection and peer-review under responsibility of the Faculty of Information Science and Technology, Universiti Kebangsaan Malaysia.

Keywords : heterodyne technique; laser diode; microwave generator; temperature condition; photodetector.

* Corresponding author: Tel.: +62-22-2501661; fax: +62-22-2534133.E-mail address: munir@ieee.org

Available online at www.sciencedirect.com

© 2013 The Authors. Published by Elsevier Ltd.Selection and peer-review under responsibility of the Faculty of Information Science & Technology, Universiti Kebangsaan Malaysia.

ScienceDirect

951 Iyon Titok Sugiarto et al. / Procedia Technology 11 ( 2013 ) 950 – 954

1. Introduction

The expanding of world telecommunication, directly or indirectly require to always continue innovating the new source of microwave as an intermediary for sending data from one base station to its central station or its opponent. Currently, dielectric resonator system is still the main technology which is usually applied as a microwave signal generator. In radar and telecommunication system, the main source of microwave signal almost came from the dielectric resonator oscillator or commonly called as DRO in which the DRO-based technique is only capable for generating microwave signal with frequency below 10GHz [1]. Hence to obtain higher frequency, the output frequency from DRO has to be doubled by using some multiplier. Signal in the upper mm-wave region is interesting for short range communication such as for radio on fiber, as well as for spectroscopy and imaging application. However, it is difficult to generate a clean signal which typically using frequency multiplier technique of low frequency signal source. Furthermore, the use of frequency multiplier technique will increase bit error rate (BER) and become disturbing phase noise of the signal.

Technological developments led to the oscillator from low frequency noise sources. One of the technologies that support it is laser technology, where the laser is a single light source is very stable frequencies with the frequency of 193THz to 1550nm wavelength. By using homodyne or heterodyne laser technique, the microwave source can be generated alternatively [2-4]. Basically, both techniques have the same principle, i.e. at least one laser diode with stable frequency as its output is required. The laser diode must be tunable for injected current and temperature. The intensity or the optical power of laser diode can be specified by providing injected current. In addition, laser is high frequency oscillator with high Q-factor, so it can be used as reference oscillator. However, the laser frequency is far from usual used microwave frequency, therefore it is difficult to count laser frequency as well as microwave signal.

2. Brief overview of laser theory

In a crystal, the discrete energy levels of the individual atom broaden into energy bands. Each quantum state of the individual atom gives rise to a certain energy band. The bonding combinations of states become the valence bands (VB) of the crystal, and the anti-bonding combinations of these states become the conduction band (CB). Theenergy difference between VB and CB is called energy gap. If the valence bands are partly filled, this material is p-type, if the conductive bands are partly filled, this material is n-type. Here Fermi level is used to label the occupation conditions of electrons in the semiconductor; it is the energy level to which electrons occupy. Fermi level (EFP) on p-type is near the valence band and EFN on the n-type is near the conductive band. When two semiconductors with different band structures are combined, a heterojunction is formed; a p-n heterojunction is called a diode. Electrons and holes transfer to the other side, because of different Fermi levels respectively. They recombine with each other, leaving the p-side with negative charge and n-side with positive charge, this region is called space charge layer (SCL) [5].

The process of recombination of excess electrons and holes with the subsequent emission of a photon lies at the heart of the diode laser. Starting from the position of recognizing that a p-n junction exists, it allows the charge to be injected into the semiconductor in order to laser action to take place. Thus, it is now possible to understand that electrons injected into the p-type material diffuse away from the junction, recombining as they go. In the main that recombination will be non irradiative, so light will be emitted, but of course this does not necessarily describe laser. A light emitting diode works very much this way, but here are crucial differences between an LED and a laser, not only in the details of the structure but also in the number of carriers injected [6].

Laser or optical wavelength is very short, and correspond to very high carrier frequency. In communication increasing the carrier frequency theoretically increases the available transmission bandwidth. As a result, frequency in optical range may have potential bandwidths of approximately 105 times that of carrier in the RF range [7]. Laser diodes have grown to a key component in modern photonics technology. Laser diode is modeling as a resonator containing plane optical wave traveling back and forth along the length of laser diode. Incident spontaneous emission light propagating to the reflection mirror is amplified by stimulated emission and comes back to initial position after a round trip inside the laser cavity. This process is subject to losses arising from light going through ordiffracting at the reflection mirrors, and scattering or absorption within an active light-emitting medium. When the total loss is higher than the gain, the light attenuates. Injected current strengthens the amplification light in the diode

952 Iyon Titok Sugiarto et al. / Procedia Technology 11 ( 2013 ) 950 – 954

laser, and when the gain and loss are balanced, initial light intensity becomes equal to the returned light intensity. This condition is referred to as threshold as expressed in (1). A laser diode oscillates above the threshold when the gain is high enough.

00 exp,

T

TITI th (1)

Optical power is one of the important parameters to characterize a laser diode. The output power is resulted by injecting current on laser system. The formula to calculate the output power can be defined by (2).

thextout IIP (2)

where dIdPext is the external quantum efficiency or the slop of laser diode quantum efficiency [8]. This optical power basically is the amount of laser diode intensity with its unit is dBm or mW.

3. Experimental setup

The type of laser diode used in the experiment is Eudyna FLD5F15CX-H9310 which has a wavelength of 1552nm. The block diagram of experimental setup is shown in Fig. 1. Laser diodes are already integrated with fiber optic-to-face type of polarizing maintenance-male FC connector types, making it easy to pair with FC-female connector on Optical Spectrum Analyzer (OSA). In the experiment, it is conducted operational temperature setting and the current distributed feedback (DFB) laser injection system. At the beginning, the injection current variations of the experiment are performed with a constant temperature is 18oC.

From the experimental data obtained value of the optical power as a function of injection current. Injection flow is regulated by a system component namely the laser driver. Laser diode currents are set with a certain amount,while the laser temperature is set at the minimum limits. In a series of DFB lasers, the temperature control that serves to regulate the temperature is applicable for laser surgery. When the laser driver is set to certain conditions, the laser driver will respond and resume commands to the peltier element. Then, the peltier element will respond by giving or reducing the temperature. Finally, the signals sent by the laser transmission are received and monitored using OSA.

Fig. 1. Block diagram of experimental setup

Laser Driver ITC 102

DFB Laser EudynaFLD5F15CX-H9310 OSA

Current control

Temperature control

953 Iyon Titok Sugiarto et al. / Procedia Technology 11 ( 2013 ) 950 – 954

4. Characterization results and discussion

Figure 2 plots the relationship between these quantities of optical power laser and its electric current. From the result, it shows that the optical power laser diode is proportional to the electrical current supplied, where the greater the current, the greater the optical power. As shown in the figure, there is a threshold current (Ith) where the number of injected current density is high enough, it will cause a stimulated recombination process, so that the optical power started to grow significantly. Threshold current starts at 14.5mA for laser temperature conditions of 18oC. When an electric current is injected more than 14.5mA, the stimulated emission is more dominant than the spontaneous emission. In this condition, the laser begins to start, while the electric current value which is below the laser diode operates as LED (Light Emitting Diode) where the inverse population does not occur, so that the spontaneous emission dominates in this area.

0.0

0.5

1.0

1.5

2.0

8 12 16 20 24 28 32 36

Current (mA)

Inte

nsity

(mW

)

18 C

Fig. 2. Injection current dependence of optical power on 18oC

1551.2

1551.4

1551.6

1551.8

1552.0

1552.2

1552.4

1552.6

1552.8

1553.0

1553.2

14 17 20 23 26 29 32temperatur (oC)

wav

elen

gth

(nm

)

I = 20 mA

I = 25 mA

I = 30 mA

I = 35 mA

I = 40 mA

Fig. 3. Characterization result of varied wavelength of laser diode to temperature change

954 Iyon Titok Sugiarto et al. / Procedia Technology 11 ( 2013 ) 950 – 954

The performed temperature variation determines the characterization of the resulting changes in wavelength. The electrical current is injected constant is 20mA, 25mA, 30mA, 35mA and 40mA, while the temperature is varied from 15 – 30°C with sampling interval of 0.2oC. From the measurement data, it is obtained a laser wavelength of 1553nm at 15°C and a laser wavelength of 1551nm at 30oC. Figure 3 plots the experimental characterization result of varied wavelength of the laser diode to the temperature change. Schematically, the laser driver will give orders to the peltier element to raise or lower the temperature when the laser wavelength laser is varied in certain circumstances. In conjunction with the results of the experiment oscillator laser system is an early stage to determine the stability of the laser wavelength. This is because of variable wavelength which will directly influence the laser frequency. Based on the results, it should be noted that the laser diode can be used as a source of microwave signal.From Fig. 3, it can be described that the increase in temperature will result in a decreasing wavelength. Based on the variation of the laser diode temperature changes from 15 – 30°C it appears that the laser diode wavelength shift of 2nm, from 1553 – 1551nm. So the value of the wavelength shift per 1°C is equal to 0.097nm/°C, or if the temperature is changed by 1°C difference in the obtained change in wavelength of 0.097nm. This value is important to determine the laser wavelength for some application.

5. Conclusion

The stability of laser diode based on distributed feedback (DFB) Eudyna FLD5F15CX-H9310 has been investigated in preliminary study for microwave generator based on experimental characterization. From the characterization results, it has been demonstrated that the optical power of laser diode is in accordance with empirical evidence, where the greater the flow of injected laser the greater the optical power generated. The relationship between the varied wavelength and the temperature is inversely proportional. When the temperature is raised from 15 – 30°C, the wavelength shifted of 2nm, i.e. from 1550 – 1552nm. So the value of wavelength shift is0.097nm/oC. Therefore, based on the result of experimental characterization it can be concluded that a laser diodecan be used for microwave frequency generator as well as for other applications.

References

[1] Widiyatmoko B, Waluyo TB, Siregar MRT. A mm-wave signal generation using laser diode: preliminary study. In: Proc. of the 10th

International Conference on Quality In Research (QIR), 2007.[2] Sun C, Huang J, Xiong B, Luo Y. Optical generation of microwave carrier with high spectral purity using integrated dual wavelength

semiconductor laser diode. In: Proc. International Conference on Indium Phosphide & Related Materials (IPRM), 2010. p. 1-4.[3] Criado AR, de Dios C, Acedo P, Carpintero G, Yvind K. Comparison of monolithic optical frequency comb generators based on passively

mode-locked lasers for continuous wave mm-wave and sub-THz generation. Journal of Lightwave Tech. 2012: 30:19. p. 3133-3141[4] Locke T, Pochet M, Usechak NG. On-off keyed microwave signal optically generated using an optically-injected Fabry-Perot semiconductor

laser. In: Proc. of Conference on Lasers and Electro-Optics (CLEO), 2012. p. 1-2.[5] Pospiech M, Liu S. Laser Dioda an Introduction, University of Hannover, 2004.[6] Gorman JO, Levi AFJ. Wavelength dependence of To in InGaAsP semiconductor laser diodes. Application Physics Lett. 1993.[7] Fukushima S, Silva CFC, Muramoto Y, Seeds AJ. Optoelectronic millimetre- wave synthesis using an optical frequency comb-generator,

optically injection locked laser, and a unitraveling-carrier photodiode. Journal of Ligthwave Tech. 2003; 21:12. p. 3043-3051[8] Sands D. Diode Laser. Bristol and Philadelphia: Institute of Physics Publishing; 2005.