Advances in OptoElectronics

Volume 2017 (2017), Article ID 1365072, 5 pages

https://doi.org/10.1155/2017/1365072

## Broadband Enhancement of Optical Frequency Comb Using Cascaded Four-Wave Mixing in Photonic Crystal Fiber

Faculty of Engineering & Technology, Multimedia University, 75450 Bukit Beruang, Melaka, Malaysia

Correspondence should be addressed to Tawfig Eltaif

Received 13 April 2017; Revised 2 June 2017; Accepted 14 June 2017; Published 12 July 2017

Academic Editor: Samir K. Mondal

Copyright © 2017 Tawfig Eltaif. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

A cascaded intensity modulator (IM) and phase modulator (PM) are used to modulate a continuous-wave (CW) laser and generate an optical frequency comb (OFC). Thus, the generated comb is utilized as an initial seed and combined with another CW-laser to generate four-wave mixing (FWM) in photonic crystal fiber (PCF). Results show that an initial flat 30 GHz OFC of 29, 55 lines within power fluctuation of 0.8 dB and 2 dB, respectively, can be achieved by setting the ratio of the DC bias to amplitude of sinusoidal signal at 0.1 and setting the modulation indices of both IM and PM at 10. Moreover, the 1st order of FWM created through 14 m of PCF has over 68 and 94 lines with fluctuation of 0.8 dB and 2 dB, respectively. Hence, the generated wavelengths of 1st left and right order of FWM can be tuned in a range from ~1500 nm to ~1525 nm and ~1590 nm to ~1604 nm, respectively.

#### 1. Introduction

Generation of four-wave mixing (FWM) in highly nonlinear low-dispersion fibers was intensively studied, so it can be utilized as an optical source for wavelength-division multiplexing (WDM) system [1–4]. Basically only two wavelengths are needed to cause the interaction between each other in nonlinear fiber to induce a nonlinear phase modulation at the beat frequency. Hence new sidebands (i.e., FWM) can be generated on both sides of the main wavelengths. Moreover, as the new lines propagate along the fiber, interaction of the lines with each other occurred, and then cascaded FWM is created. An optical feedback to the input port scheme was proposed to enhance the cascaded four-wave mixing (CFWM) generation [5]. To generate frequency comb within broad bandwidth over highly nonlinear fiber (HNLF), the absolute value of the HNLF dispersion should not exceed 1 ps/nm/km within the comb range. In 2003, Okuno et al. have successfully fabricated flattened-HNLF with properties that satisfied the simplified dispersive requirement; unfortunately it failed to generate continuous-wave- (CW-) seeded frequency comb [6]. In 2012, Myslivets et al. successfully managed to generate broadband optical frequency combs based on cascaded FWM in highly nonlinear fiber low-power, continuous-wave seeds, but the flatness is too poor [7]. High sensitivity in highly nonlinear fiber (HNLF) was achieved when the signal’s wavelength is positioned at the band edge of the modulation instability (MI) spectrum generated by an intense degenerate four-wave mixing (FWM) pump [8]. In 2008, an investigation was conducted by using conventional fibers and ultraflattened dispersion photonic crystal fibers to generate 118 FWM products over bandwidth of 300 nm [2]. In 2009, an optimized technique using three-pumps with unequally spaced frequencies was implemented to generate frequency combs by four-wave mixing in highly nonlinear low-dispersion fibers [3]. Zhang et al. used highly nonlinear photonic crystal fiber (PCF) to generate wavelength-tunable optical pulse train based on four-wave mixing [1]. An alternative configuration based on nonlinear effect of intensity and phase modulators was implemented to generate OFC [9–11]. Unfortunately, using configuration with small number of phase modulators led to either poor flatness over large bandwidth or a limited number of lines over small flat bandwidth, the only way to achieve wide flat bandwidth by cascading many modulators, which is very expensive. Therefore, in 2014 a very simple configuration consists of intensity and phase modulators, two lasers sources and highly nonlinear fiber, was investigated and showed that the modulators sidebands were doubled after the highly nonlinear fiber and over 100 lines were achieved [12, 13]. In terms of application, optical frequency comb is very useful for high-repetition-rate pulse train generation [14] and for injection locking of widely separated lasers [15]. Such OFC can reduce the cost of WDM system, where many wavelengths can be generated and each can be used to carry single user’s data [16]. In addition, by using an appropriate optical bandpass filters to select certain sidebands from OFC comb and beating any two wavelengths in photodetector will generate millimeter-wave signal that can be used for 5G application [17].

Hence, in this paper, with advantages of nonlinear optics effect of intensity modulator (IM) followed by phase modulator (PM) and FWM caused by two laser sources over 14 m of photonic crystal fiber cascaded four-wave mixing was achieved. Furthermore, the spacing between the two laser sources and nonlinear coefficient of PCF were investigated to choose the optimum values to increase the bandwidth of the FWM as compared to the initial comb generated by a cascade of IM and PM.

#### 2. System Setup

Figure 1 shows the configuration of the optical frequency comb, which consists of a cascade of one intensity modulator followed by one phase modulator. Both of the modulators were driven by a sinusoidal signal with a frequency of 30 GHz, which modulated a continuous-wave (CW) laser wavelength centered at 1540 nm. Then, the output combined with another laser source centered at wavelength 1568 nm. Both laser sources are set at 15 dBm. Hence, the initial comb was generated as shown in inset (a), Figure 1. A cascade of two optical amplifiers were added to increase the power of the generated lines. Then, the initial comb passes through 14 m of photonic crystal fiber (PCF), which helps to generate more lines in terms of FWM as shown in inset (b), Figure 1. I set PCF parameters as in [18], which has the following parameters: linear losses = 0.2 dB/km, group velocity dispersion ps/nm/km, slope ps/nm^{2}/km, and nonlinear coefficient W^{−1} km^{−1}.