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Wideband optical phase modulator half-wave voltage measurement technique based on phase modulation optical link
Optik 124 (2013) 5385–5387
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.de/ijleo
Wideband optical phase modulator half-wave voltage measurement technique based on phase modulation optical link Quanyi Ye, Chun Yang ∗ School of Electronic Science and Engineering, Southeast University, Nanjing 210096, China
a r t i c l e
i n f o
Article history: Received 26 October 2012 Accepted 22 March 2013
Keywords: Optical communication Optical link Phase modulator Half-wave voltage
a b s t r a c t We measured the half-wave voltage V of LiNbO3 phase modulators in the broadband frequency range by analyzing the gain of phase modulation interference demodulation optical link. This is a new high practical value measurement method for half-wave voltage of LiNbO3 phase modulators in wideband frequency range, and can accurately predict the nonlinear frequency characteristics of phase modulation optical link. © 2013 Elsevier GmbH. All rights reserved.
1. Introduction The LiNbO3 phase modulation has found great potential in recent application, not only eliminates the bias circuit, but also provides more linear conversion of input voltage to optical phase [1,2]. The half-wave voltage (V ) is the voltage required to produce a radians phase shift, and is the important characterize for the electro-optic phase modulators. The effect which usually appears in microwave range is known as it influences system performance of analog optical fiber link by LiNbO3 phase modulator [3,4]. Different techniques have been proposed up today: one typically method is observed optical spectrum with an optical spectrum analyzer (OSA) [4–6], another method is converts the intensity variations into an electrical signal with the photodetector and then measured with a spectrum analyzer [7,8]. An alternative method is insert a tunable unbalanced Mach–Zehnder interferometer and measure the effective half-wave voltages of phase modulators [9]. However these methods are limited in bandwidth of the measuring instruments, the line-width of the optical sources and the point-by-point measurements are slow for a full investigation of the frequency behavior. In this paper, we measure the half-wave voltage of a LiNbO3 phase modulators using measuring the gain in a phase modulation optical link with non-balanced Mach–Zehnder interferometer. We
∗ Corresponding author. E-mail address: [email protected] (C. Yang). 0030-4026/$ – see front matter © 2013 Elsevier GmbH. All rights reserved. http://dx.doi.org/10.1016/j.ijleo.2013.03.142
present the theoretical principle of the method used to evaluate the half-wave voltage at first. The experiments and experimental show that this broadband half-wave voltage measurement has a simple structure, easy operation and can tune by a fiber jumper.
2. Measurement principle The schematic diagram of the half-wave voltage measurement system is shown in Fig. 1. The basic elements of a phase-modulated link comprising a laser source (in this case a distributed-feedback laser (DFB)), RF source (RF), LiNbO3 phase modulator (PM) and photodetector (PD). The light, which is phase-modulated by the RF source form vector network analyzer, is divided in MZ Interferometer. Then the FM-AM transformed signal is obtained at the photodetector. The gain of phase modulation optical link can be detected directly by the network analyzer.
2.1. The gain of phase modulation optical link In our experiment, a laser with a center wavelength of 1550 nm is used. To analyze the system we assume an electric field from
the laser of the form Ein = 2k2 Pin ejω0 t , where Pin is the optical power, ω0 is the optical radian frequency, and k is a scale factor relating electric field amplitude to square root of optical power. The drive voltage is given by Vin (t) = Vm sin(ωm t), where ωm is the angular of the drive signal. For an ideal phase modulator and
5386
Q. Ye, C. Yang / Optik 124 (2013) 5385–5387
dependence on ωm /2. In the other word, the frequencies at which the response is a maximum are fk =
1 1 + (k − 1) × , 2×
and f1 =
1 , 2×
k is a constant. (6)
Fig. 1. Configuration of half-wave voltage measurement system based phase modulation interferometric demodulation optical link.
In the above equation, the time delay is defined as: = nL/c, while n is the refractive index for fiber, L is optical path difference, c is the light speed 3 × 108 m/s. fk =
c c +k× 2 × n × L n × L
and f1 =
c 2 × n × L
(7)
From the above equation, the first modulation frequency at the peak gain is inversely proportional to the optical path difference. The modulation frequency is smaller when the optical path difference is longer. The number of the gain peak modulation frequency is n=
F × n × L F = c f
(8)
That means the number of the gain peak frequency is proportional to the optical path difference when optical fiber refractive index and the light speed is regular. We can tuning the frequency number of the gain peak by insert the different jumper, then tuning the definition for the half-wave voltage of the phase modulator by Eq. (5). 3. Experiment and discussion
Fig. 2. Half-wave voltage curve of the phase modulator.
photodiode, the photocurrent output of the photodiode is given by I(t)
= Idc 1 + sin 2() sin (ωm (t − )) sin = Idc 1 + 2J1 2() sin
2
ωm
2
ωm
sin ωm t −
2
+2J3 2() sin
In the above equation, Idc = apin /2 is the total dc photocurrent, a is the total attenuation of link, is the response of the photo detector, Pin is the input optical power. Zin , Zout are load resistance. = VRF /V is the phase-shift amplitude through the phase modulator, where V is the voltage required to produce a radians phase shift. The being the differential time delay between the two arms of the MZI. So, the output current of RF for a class of Bessel functions is
IRF = 2Idc J1 2() sin
2
ωm
sin ωm t −
2
(2)
Notice that the maximum output power for a phase modulation optical link given by [10] Pout
max
2 2 = 2Idc J1 ()Zout
(3)
From Eq. (3), the small-signal gain factor for a phase modulation optical link given is defined as (3): Gmax =
2 2 Z Z 4Idc in out
V2
(4)
So the V of the phase modulator is
V = Idc
10
Zin Zout Gmax (dB)/10
(5)
2.2. The gain peak frequency tuning by fiber jumper The periodic nature of the output power for a phase modulation optical link shown in Eq. (3) should be apparent in the functional
In these experiments a narrow-linewidth distributed feedback (DFB) semiconductor laser is injected into a LiNbO3 phase modulators. The gain of link was measured using a network analyze.
2
ωm
sin 3ωm t −
2
+ ···
(1)
In these experiments, the total attenuation of link is 6.5 dB, the response photo detector is 0.7 A/W, the input optical power is 11 dBm, the total dc photocurrent Idc = a pin /2. To our knowledge, these experiments represent gain peak in the phase-modulated optical link with network k analyze. Fig. 2 illustration is the measured gain spectra with network analysis. It can be seen that the gain has been a downward trend as the frequency increases, mainly due to the phase modulator and detector frequency response. Fig. 2 shows the half-wave voltage as a function of frequency using Eq. (5) of previous section. The effective half-wave voltage is increasing by frequencies, starting from 4.7 V at 0.5 GHz to 9 V at 20 GHz. The same method is applied at same frequencies in the range of 1–10 GHz for delays of 0.002 m. From Eqs. (4) and (8), there are two peak gains modulation frequencies in the 20 GHz frequency range. Fig. 3 shows the plots of the link gain. There are calculated without half-wave voltage correct (dashed lines), measured (solid line) and calculated with half-wave voltage correct (circles). As can be seen by the experimental curve (the solid line), the gain of the interferometer demodulation of phase modulated optical link cyclical transformation, which is consistent with the theoretical analysis by Eq. (3) by the solid line curve. It can also be seen within each gain peak showing a decreasing trend because the frequency of the phase modulator and detector response. Compare the measured (solid line) and calculated with half-wave voltage correct date, the two is consistent. Phase modulator half-wave voltage revised value of the gain curve and the measured gain data that is taken in good agreement. This also proves the effectiveness of this method of application.
Q. Ye, C. Yang / Optik 124 (2013) 5385–5387
5387
frequency dependencies influence on the quality of the modulator output. References
Fig. 3. Link gain curve calculated with half-wave voltage correct (circles), without half-wave voltage correct (dashed lines), and measured data (solid line).
4. Conclusion We measured the frequency response of half-wave voltage of LiNbO3 phase modulators in wideband frequency range, by measuring the gain in a phase optical modulation link. This method is simple structure and easy operation. Future work includes how the
[1] L. Lan, L.Z. Shi, Z. Xian-Min, J. Xiao-Feng, C. Hao, Performance improvement of phase modulation with interferometric detection through low-biasing, J. Electromagn. Waves Appl. 24 (1) (2010) 123–132. [2] F. Bucholtz, V.J. Urick, M.S. Rogge, K.J. Williams, Performance of analog photonic links employing phase modulation, in: Coherent Optical Technologies and Applications (COTA), 2006. [3] Y. Shi, L. Yan, A.E. Willner, High-speed electrooptic modulator characterization using optical spectrum analysis, IEEE/OSA J. Lightwave Technol. 21 (10) (2003) 2358–2367. [4] L.-S. Yan, A.E. Willner, Yongqiang Shi, Graphical solution for RF half-wave voltage and chirp parameter of electrooptic modulators using optical spectrum analysis, IEEE Photon. Technol. Lett. 17 (7) (2005) 1486–1488. [5] T. Kawanishi, T. Sakamoto, A. Chiba, H. Toda, A. Enokihara, H. Murata, Measurement of Mach-Zehnder Interferometer Based Modulators by Using Optical Spectrum Analysis (in Japanese), IEICE Techical Report OPE2008-99, 2008. [6] Y. Tsuchiya, Y. Ogiso, S. Shinada, S. Nakajima, T. Kawanishi, H. Nakajima, Evaluation of frequency characteristics of LiNbO3 phase modulator in low frequency range, in: Annual Meeting of IEICE Society C-3-77, 2009 (in Japanese). [7] Kimihiro Tok, Dai Uesaka, H. Takahiro Yamazaki Iwata, M. Yoshikawa, H. Toda, T. Kawanishi, Frequency Response Measurement of Half-Wave Voltage and Chirp Parameter of LiNbO3 Intensity Modulators in Low Frequency Range, MWP, 2011, pp. 354–356. [8] E. H.W. Chan, R.A. Minasian, A new optical phase modulator dynamic response measurement technique, J. Lightwave Technol. 26 (August (16)) (2008). [9] J. Hauden, H. Porte, An Alternative Method for High Frequency Effective V Measurements of Phase Modulators, MWP, 2011, pp. 161–164. [10] V.J. Urick, B. Frank, J.D. McKinney, P.S. Devgan, Anthony L. Campillo, J.L. Dexter, K.J. Williams, Long-haul analog photonics, J. Lightwave Technol. 29 (April (8)) (2011) 1182–1205.
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.de/ijleo
Wideband optical phase modulator half-wave voltage measurement technique based on phase modulation optical link Quanyi Ye, Chun Yang ∗ School of Electronic Science and Engineering, Southeast University, Nanjing 210096, China
a r t i c l e
i n f o
Article history: Received 26 October 2012 Accepted 22 March 2013
Keywords: Optical communication Optical link Phase modulator Half-wave voltage
a b s t r a c t We measured the half-wave voltage V of LiNbO3 phase modulators in the broadband frequency range by analyzing the gain of phase modulation interference demodulation optical link. This is a new high practical value measurement method for half-wave voltage of LiNbO3 phase modulators in wideband frequency range, and can accurately predict the nonlinear frequency characteristics of phase modulation optical link. © 2013 Elsevier GmbH. All rights reserved.
1. Introduction The LiNbO3 phase modulation has found great potential in recent application, not only eliminates the bias circuit, but also provides more linear conversion of input voltage to optical phase [1,2]. The half-wave voltage (V ) is the voltage required to produce a radians phase shift, and is the important characterize for the electro-optic phase modulators. The effect which usually appears in microwave range is known as it influences system performance of analog optical fiber link by LiNbO3 phase modulator [3,4]. Different techniques have been proposed up today: one typically method is observed optical spectrum with an optical spectrum analyzer (OSA) [4–6], another method is converts the intensity variations into an electrical signal with the photodetector and then measured with a spectrum analyzer [7,8]. An alternative method is insert a tunable unbalanced Mach–Zehnder interferometer and measure the effective half-wave voltages of phase modulators [9]. However these methods are limited in bandwidth of the measuring instruments, the line-width of the optical sources and the point-by-point measurements are slow for a full investigation of the frequency behavior. In this paper, we measure the half-wave voltage of a LiNbO3 phase modulators using measuring the gain in a phase modulation optical link with non-balanced Mach–Zehnder interferometer. We
∗ Corresponding author. E-mail address: [email protected] (C. Yang). 0030-4026/$ – see front matter © 2013 Elsevier GmbH. All rights reserved. http://dx.doi.org/10.1016/j.ijleo.2013.03.142
present the theoretical principle of the method used to evaluate the half-wave voltage at first. The experiments and experimental show that this broadband half-wave voltage measurement has a simple structure, easy operation and can tune by a fiber jumper.
2. Measurement principle The schematic diagram of the half-wave voltage measurement system is shown in Fig. 1. The basic elements of a phase-modulated link comprising a laser source (in this case a distributed-feedback laser (DFB)), RF source (RF), LiNbO3 phase modulator (PM) and photodetector (PD). The light, which is phase-modulated by the RF source form vector network analyzer, is divided in MZ Interferometer. Then the FM-AM transformed signal is obtained at the photodetector. The gain of phase modulation optical link can be detected directly by the network analyzer.
2.1. The gain of phase modulation optical link In our experiment, a laser with a center wavelength of 1550 nm is used. To analyze the system we assume an electric field from
the laser of the form Ein = 2k2 Pin ejω0 t , where Pin is the optical power, ω0 is the optical radian frequency, and k is a scale factor relating electric field amplitude to square root of optical power. The drive voltage is given by Vin (t) = Vm sin(ωm t), where ωm is the angular of the drive signal. For an ideal phase modulator and
5386
Q. Ye, C. Yang / Optik 124 (2013) 5385–5387
dependence on ωm /2. In the other word, the frequencies at which the response is a maximum are fk =
1 1 + (k − 1) × , 2×
and f1 =
1 , 2×
k is a constant. (6)
Fig. 1. Configuration of half-wave voltage measurement system based phase modulation interferometric demodulation optical link.
In the above equation, the time delay is defined as: = nL/c, while n is the refractive index for fiber, L is optical path difference, c is the light speed 3 × 108 m/s. fk =
c c +k× 2 × n × L n × L
and f1 =
c 2 × n × L
(7)
From the above equation, the first modulation frequency at the peak gain is inversely proportional to the optical path difference. The modulation frequency is smaller when the optical path difference is longer. The number of the gain peak modulation frequency is n=
F × n × L F = c f
(8)
That means the number of the gain peak frequency is proportional to the optical path difference when optical fiber refractive index and the light speed is regular. We can tuning the frequency number of the gain peak by insert the different jumper, then tuning the definition for the half-wave voltage of the phase modulator by Eq. (5). 3. Experiment and discussion
Fig. 2. Half-wave voltage curve of the phase modulator.
photodiode, the photocurrent output of the photodiode is given by I(t)
= Idc 1 + sin 2() sin (ωm (t − )) sin = Idc 1 + 2J1 2() sin
2
ωm
2
ωm
sin ωm t −
2
+2J3 2() sin
In the above equation, Idc = apin /2 is the total dc photocurrent, a is the total attenuation of link, is the response of the photo detector, Pin is the input optical power. Zin , Zout are load resistance. = VRF /V is the phase-shift amplitude through the phase modulator, where V is the voltage required to produce a radians phase shift. The being the differential time delay between the two arms of the MZI. So, the output current of RF for a class of Bessel functions is
IRF = 2Idc J1 2() sin
2
ωm
sin ωm t −
2
(2)
Notice that the maximum output power for a phase modulation optical link given by [10] Pout
max
2 2 = 2Idc J1 ()Zout
(3)
From Eq. (3), the small-signal gain factor for a phase modulation optical link given is defined as (3): Gmax =
2 2 Z Z 4Idc in out
V2
(4)
So the V of the phase modulator is
V = Idc
10
Zin Zout Gmax (dB)/10
(5)
2.2. The gain peak frequency tuning by fiber jumper The periodic nature of the output power for a phase modulation optical link shown in Eq. (3) should be apparent in the functional
In these experiments a narrow-linewidth distributed feedback (DFB) semiconductor laser is injected into a LiNbO3 phase modulators. The gain of link was measured using a network analyze.
2
ωm
sin 3ωm t −
2
+ ···
(1)
In these experiments, the total attenuation of link is 6.5 dB, the response photo detector is 0.7 A/W, the input optical power is 11 dBm, the total dc photocurrent Idc = a pin /2. To our knowledge, these experiments represent gain peak in the phase-modulated optical link with network k analyze. Fig. 2 illustration is the measured gain spectra with network analysis. It can be seen that the gain has been a downward trend as the frequency increases, mainly due to the phase modulator and detector frequency response. Fig. 2 shows the half-wave voltage as a function of frequency using Eq. (5) of previous section. The effective half-wave voltage is increasing by frequencies, starting from 4.7 V at 0.5 GHz to 9 V at 20 GHz. The same method is applied at same frequencies in the range of 1–10 GHz for delays of 0.002 m. From Eqs. (4) and (8), there are two peak gains modulation frequencies in the 20 GHz frequency range. Fig. 3 shows the plots of the link gain. There are calculated without half-wave voltage correct (dashed lines), measured (solid line) and calculated with half-wave voltage correct (circles). As can be seen by the experimental curve (the solid line), the gain of the interferometer demodulation of phase modulated optical link cyclical transformation, which is consistent with the theoretical analysis by Eq. (3) by the solid line curve. It can also be seen within each gain peak showing a decreasing trend because the frequency of the phase modulator and detector response. Compare the measured (solid line) and calculated with half-wave voltage correct date, the two is consistent. Phase modulator half-wave voltage revised value of the gain curve and the measured gain data that is taken in good agreement. This also proves the effectiveness of this method of application.
Q. Ye, C. Yang / Optik 124 (2013) 5385–5387
5387
frequency dependencies influence on the quality of the modulator output. References
Fig. 3. Link gain curve calculated with half-wave voltage correct (circles), without half-wave voltage correct (dashed lines), and measured data (solid line).
4. Conclusion We measured the frequency response of half-wave voltage of LiNbO3 phase modulators in wideband frequency range, by measuring the gain in a phase optical modulation link. This method is simple structure and easy operation. Future work includes how the
[1] L. Lan, L.Z. Shi, Z. Xian-Min, J. Xiao-Feng, C. Hao, Performance improvement of phase modulation with interferometric detection through low-biasing, J. Electromagn. Waves Appl. 24 (1) (2010) 123–132. [2] F. Bucholtz, V.J. Urick, M.S. Rogge, K.J. Williams, Performance of analog photonic links employing phase modulation, in: Coherent Optical Technologies and Applications (COTA), 2006. [3] Y. Shi, L. Yan, A.E. Willner, High-speed electrooptic modulator characterization using optical spectrum analysis, IEEE/OSA J. Lightwave Technol. 21 (10) (2003) 2358–2367. [4] L.-S. Yan, A.E. Willner, Yongqiang Shi, Graphical solution for RF half-wave voltage and chirp parameter of electrooptic modulators using optical spectrum analysis, IEEE Photon. Technol. Lett. 17 (7) (2005) 1486–1488. [5] T. Kawanishi, T. Sakamoto, A. Chiba, H. Toda, A. Enokihara, H. Murata, Measurement of Mach-Zehnder Interferometer Based Modulators by Using Optical Spectrum Analysis (in Japanese), IEICE Techical Report OPE2008-99, 2008. [6] Y. Tsuchiya, Y. Ogiso, S. Shinada, S. Nakajima, T. Kawanishi, H. Nakajima, Evaluation of frequency characteristics of LiNbO3 phase modulator in low frequency range, in: Annual Meeting of IEICE Society C-3-77, 2009 (in Japanese). [7] Kimihiro Tok, Dai Uesaka, H. Takahiro Yamazaki Iwata, M. Yoshikawa, H. Toda, T. Kawanishi, Frequency Response Measurement of Half-Wave Voltage and Chirp Parameter of LiNbO3 Intensity Modulators in Low Frequency Range, MWP, 2011, pp. 354–356. [8] E. H.W. Chan, R.A. Minasian, A new optical phase modulator dynamic response measurement technique, J. Lightwave Technol. 26 (August (16)) (2008). [9] J. Hauden, H. Porte, An Alternative Method for High Frequency Effective V Measurements of Phase Modulators, MWP, 2011, pp. 161–164. [10] V.J. Urick, B. Frank, J.D. McKinney, P.S. Devgan, Anthony L. Campillo, J.L. Dexter, K.J. Williams, Long-haul analog photonics, J. Lightwave Technol. 29 (April (8)) (2011) 1182–1205.