A time-domain photonic arbitrary waveform generator

A time-domain photonic arbitrary waveform generator Jinxin Liao, He Wen, Xiaoping Zheng,* Hanyi Zhang, and Bingkun Zhou State Key Laboratory on Integrated Optoelectronics / Tsinghua National Laboratory for Information Science and Technology, Department of Electronic Engineering, Tsinghua University, Beijing, 100084, China *[email protected]

Abstract: A time domain photonic arbitrary waveform generator (PAWG) scheme based on multi-wavelength optical differential quadrature phase shift keying modulation in combination with differential detection is proposed and experimentally demonstrated. The time domain method shows advantages of large time-bandwidth product, good flexibility, fast waveform refreshing rate, and high waveform quality over the frequency domain method. In contrast with other proposed time domain PAWGs or photonic digital-to-analog converters, our PAWG proposal shows a greater dynamic range and a larger noise margin due to its bipolar output, and possesses good scalabilities both in resolution and sampling rate. Assisted with the integration technology, this PAWG presents a good prospect for broad range practical applications in future. ©2012 Optical Society of America OCIS codes: (070.2025) Discrete optical signal processing; (070.6020) Continuous optical signal processing; (060.5625) Radio frequency photonics; (060.4510) Optical communications.

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Received 21 Mar 2012; revised 7 May 2012; accepted 12 May 2012; published 18 May 2012 21 May 2012 / Vol. 20, No. 11 / OPTICS EXPRESS 12631

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1. Introduction High speed arbitrary waveform generation has grown as an important area which finds extensive commercial and military applications, such as testing, large capacity communications, high speed signal processing, and military defense electronics etc. As the optical domain processing technology possesses inherent advantages of large bandwidth, high frequency and immunity to electromagnetic interferences which are difficult for the allelectronic approach to achieve, the photonic arbitrary waveform generator (PAWG) has received significant attention. At present, most of the existing PAWG schemes employ the frequency-domain method known as a fourier-transform based technique. This method allows the amplitude and phase of discrete optical spectral lines to be independently controlled and enables arbitrary waveform to be synthesized in a time aperture [1–8]. In these frequencydomain PAWG apparatus [1–3], a grating followed by a spatial light modulator (SLM) with a large number of spectral controlled elements is usually implemented as an optical pulse shaper. It shows good performance in generating complex radio frequency waveform but with two significant drawbacks. One is that the waveform refreshing time is limited by the response time of the SLM (~10ms). This problem can be released by utilizing the time multiplexing technique [4–7] or replacing the SLM with a much more short response time shaper, such as the electro-optical modulator array [8]. The other one significant drawback is that the maximum time aperture of the output temporal waveform (~2ns) directly associated with the lowest attainable frequency, is determined by the minimum resolution (~1GHz) of the optical pulse shaper employed in the system [9]. The time multiplexing technique can increase the time aperture but only to some extent. The reason is that large numbers of the multiple branches in time-multiplexing based schemes are required to achieve a PAWG capable of fast and flexible waveform refreshing along with infinite waveform time aperture. The number corresponding to the ratio of the waveform refreshing time and the waveform time aperture is tremendous (~105). It is very difficult to expand so many multiple branches to meet the requirement. However, the PAWG employing the time-domain method which is similar to the traditional electrical AWG does not have these problems [10, 11]. The time-domain PAWG generates samples to approximate the target waveform at each temporal point. The waveform time aperture is determined by the product of the memory depth and the sampling interval, which can be long enough to several milliseconds. It can be much longer with the real-time sequencing technique. Thus, the time-domain method can provide a larger time-bandwidth product compared with the frequency-domain method. Furthermore, the flexibility, the quality and the refreshing rate of the time-domain arbitrary waveform generation are excellent. The time-domain PAWG can be regarded as a photonic digital-to-analog converter (PDAC). But

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Received 21 Mar 2012; revised 7 May 2012; accepted 12 May 2012; published 18 May 2012 21 May 2012 / Vol. 20, No. 11 / OPTICS EXPRESS 12632

so far, all the proposed PDAC are unipolar, with only positive value [10–16]. From the perspective of dynamic range and noise margin, a bipolar PDAC (BPDAC) may have an advantage over the unipolar PDAC and can achieve higher resolution more easily. This is illustrated by comparing the constellation diagram of the bipolar output against that of the unipolar output [17]. In [18], we have recently put forward a 2N-bit BPDAC based on multi-wavelength optical differential quadrature phase shift keying (ODQPSK) modulation in combination with differential detection. In this paper, this 2N-bit BPDAC is applied to achieve a time domain PAWG with good flexibility, fast waveform refreshing rate and high waveform quality. This PAWG shows three attractive features. The first is that the bipolar output brings about a greater dynamic range and a larger noise margin compared with the unipolar output of other proposed time domain PAWGs or PDACs. The second is that it has good scalabilities both in the sample resolution and the sampling rate. The sample resolution can be increased by using multi-wavelength laser array, and the sampling rate can be increased by using timemultiplexing technique. The third is that it consisting of a multi-wavelength DQPSK transmitter and a DPSK receiver is fully compatible with the mature popular DQPSK system. It can be easily integrated on a chip or constructed with the available photonic integrated circuit (PIC). We set up a proof-of-principle PAWG experiment with 4-bit resolution at 2.5GS/s to demonstrate this scheme. The generated arbitrary waveforms are very close to the ideal waveforms. The average spurious-free dynamic range (SFDR) results of single tone and two tone signals are 31.4dB and 27.9dB, respectively. We also experimentally verify the timemultiplexing PAWG with multiple sampling rates. Furthermore, we stress some specifications of the essential components in this PAWG, and give a summary on this PAWG proposal. 2. Operation principle Figure 1(a) shows the architecture of the PAWG with 2N-bit resolution. It is composed of two main parts, the multi-wavelength ODQPSK modulation part and the differential demodulation part. In the multi-wavelength ODQPSK modulation part, N channels of independent light with different wavelengths pass through N dual parallel MZMs (DP-MZM) respectively. Each of the DP-MZM is driven by two channels of non-return-zero (NRZ) digital signal, generating one ODQPSK signal. Combined the N branches together, the multi-wavelength ODQPSK signal is delivered to the demodulation part where a 1-bit delay-time interferometer (DI) acts as a differential demodulator and its two outputs are balanced detected. We first illustrate how the samples with 2-bit resolution are generated by using single wavelength laser, then we detail the implementation of the PAWG with 2N-bit resolution and present the timemultiplexing technique assisted PAWG. (b)

(a) I1

1

P

D2n D2 n 1 D4 D3 D2 D1

18.4

DP-MZM1



precoder

90

Re{E}

Vout 11

Q1 I2

2

4P

DP-MZM2

Qn I n

10

Q2 I 2 Q1 I1

……

Im{E}

90

t

Q2

Vout

T

n

n -1

4 P

In

DP-MZMn 90

Qn

 DI

LPF BD

01

00

Fig. 1. (a)Architecture of the PAWG with 2N-bit resolution. (b) The changing form of the analog output Vout varies with the phase shift ∆φ in the 2-bit PAWG. When ∆φ18.4°, Vout presents bipolar four-equally-spaced discrete values.

#165140 - $15.00 USD (C) 2012 OSA

Received 21 Mar 2012; revised 7 May 2012; accepted 12 May 2012; published 18 May 2012 21 May 2012 / Vol. 20, No. 11 / OPTICS EXPRESS 12633

2.1 Principle of PAWG with 2-bit resolution based on ODQPSK modulation in combination with differential detection In the 2-bit PAWG using a single wavelength, the ODQPSK signal can be expressed as E  t  ei , where    4,3 4,5 4,7 4 . After differential detection, the amplitude of the output sample Vout is given by:

Vout  E1  t  T  ei1  E2  t  ei2 ei  E1  t  T  ei1  E2  t  ei2 ei 2

2

 4 E1  t  T  E2*  t  cos      ,    2  1   0,  2,  ,3 2

(1)

where E1  t  T  ei1 is the ODQPSK signal delayed by 1 bit period in the longer arm of DI,

E2  t  ei2 ei is the ODQPSK signal in the short arm of DI, ∆φ denotes the phase shift between the two arms, and T is the digital bit duration. From Eq. (1), it can be inferred that Vout varies with the phase shift ∆φ in a form of cosine function, where ∆θ determines the initial phase of the function. Figure 1(b) illustrates this phenomenon. As long as the phase shift ∆φ meets the condition cos  sin   1 3 or3 , Vout presents bipolar four-equallyspaced discrete values, In Fig. 1(b),   18.4 is taken for example. As Vout is directly associated with ∆θ which denotes the changing form between the adjacent input digital codes, a precoder is required to map the input digital codes to the bipolar four-equally-spaced discrete values. The coding algorithm of the precoder is given in Eq. (2), and the one-to-one mapping between the input and the output is shown in Table 1. Thus, a 2-bit PAWG is achieved. Profiting from the optical phase modulation combined with balanced detection, this PAWG generates bipolar samples, and shows advantages of a greater dynamic range and a larger noise margin over the unipolar PAWG or PDAC based on the intensity modulation together with direct detection [12–14]. The reason is that the minimal distance between adjacent two symbols of the phase modulated signal is larger than that of the intensity modulated signal. This advantage is analogous to the 3dB optical signal-to-noise ratio (OSNR) advantage that the phase modulation format owns over the intensity modulation format in optical communication system.

  I n  D2 n 1  D2 n   D2 n 1  I n -1    D2 n 1  D2 n    D2 n  Qn -1    Qn  D2 n 1  D2 n   D2 n  Qn -1    D2 n 1  D2 n    D2 n 1  I n -1 

(2)

Table 1. The One-To-One Mapping Between the Input and the Output ∆θ

Input digital codes 00 01 10 11

0 3π/2 π/2 π

Output analog value 0 1 2 3

2.2 Implementation of PAWG with 2N-bit resolution The resolution of this PAWG can be scaled from 2-bit to 2N-bit by using N channels of incoherent light with different wavelengths, where the optical power ratio is set as 1: 4 : : 4N 1 . Similar weighted and summing methods have been reported in [12–14]. For multi-wavelength laser array, there are some requirements on the frequency difference ∆f between any two light waves that should be specified. First, ∆f should be an integral multiple of the free spectral range (FSR) of DI which is equal to 1/T, to ensure that each wavelength light wave obtains the same phase shift of ∆φ in DI, since the transmission function of the DI is periodic and the phase shift ∆φ is directly associated with the lightwave frequency [17].

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Received 21 Mar 2012; revised 7 May 2012; accepted 12 May 2012; published 18 May 2012 21 May 2012 / Vol. 20, No. 11 / OPTICS EXPRESS 12634

Transm. [dB]

Second, ∆f should be greater than the detector’s bandwidth to ensure that the beating noise of any two light waves is removed. Figure 2 illustrates these requirements. The optical power ratio of different wavelengths can also be realized by using a multi-wavelength laser array with equal optical power of each wavelength followed by a special weighted coupler with split ratio of 1: 4 : : 4N 1 .

4n -1 P

Δφ 4P

P

1T

Δf

18.4

Frequency Fig. 2. Requirements on the multi-wavelength laser array.

2.3 Time multiplexing PAWG with 2N-bit resolution Time Multiplexing Unit (× m) 1 90

2 90



1

3

0

2 13

2

12 3

8 m

TMU1 4n -1 P P

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m 90

time- and wavelengthinterleaved short pulse generator

0

8 6

4

TMU2

T

Output

TMUn



DI

LPF BD

Fig. 3. Architecture of the time multiplexing PAWG with 2N-bit resolution.

With the optical domain time passive multiplexing method which has advantages of low loss, stable delay time, and immunity to electromagnetic interferences etc, the multi-level optical short pulse can be easily temporal interleaved to increase the sampling rate of this PAWG. The architecture of the 2N-bit PAWG with m multiple expanded sampling rate is presented in Fig. 3. In the apparatus, the multi-wavelength time-interleaved short pulse generator with very small time jitter (