1. Introduction

Coherent detection has been studied extensively in optical fiber communications and is getting renewed attention because it could meet increasing bandwidth demands by maximizing the spectral efficiency of wavelength division multiplexing (WDM) systems. In coherent detection, the phase of the optical signal can be used as well as intensity in various modulation methods, while phase information is lost in direct detection. Also, coherent detection can improve the signal to noise ratio (SNR) because it uses a local oscillator (LO) whose optical power is much larger than the signal power. However, the relative intensity noise (RIN) of the LO increases as the LO optical power increases, and SNR starts to decrease when the contribution of RIN becomes comparable to shot noise [1,2]. At this high optical LO power level, balanced coherent detection should be used to avoid SNR degradation caused by the laser RIN and amplified spontaneous emission (ASE) noise from an erbium-doped fiber amplifier used as the local oscillator [3], and thus achieve shot-noise limited system performance when used in an optical link. For coherent detection in fiber optic applications, development of balanced photodetectors with high speed and high saturation photocurrents is especially important. Balanced photodetectors monolithically integrated with optical waveguides using velocity matched distributed traveling-wave photodetectors (TWPDs) [4,5] and waveguide photodiodes [6,7] have been demonstrated. All these previously reported balanced PDs were integrated with semiconductor optical waveguides. In [6], a semiconductor multimode interference (MMI) coupler was integrated to achieve further on-chip integration of optical devices. However, integrating semiconductor waveguide elements on the same wafer requires more complex epitaxial layer growth and makes device fabrication complicated. Polymer optical waveguide elements provide an attractive alternative to using semiconductor materials because polymer waveguide elements, such as MMI couplers [8] and delay lines [9], and polymer modulators [10] have been demonstrated and can be integrated monolithically with photodetectors. For on-wafer optoelectronic device integration, polymer material can be widely used for future photonic circuit integration since polymer optical waveguide elements can be laid easily on a wafer. Although monolithic integration of polymer optical waveguides and photodetectors has been reported [11,12], the previously reported photodetectors were integrated with multimode polymer optical waveguides and were not designed as balanced photodetectors for coherent detection.

In this report, we demonstrate balanced coherent detection using single mode benzocyclobutene (BCB) polymer optical waveguides laid on distributed balanced TWPDs for the first time. This demonstration shows the possibility for more complicated photonic integrated circuits using polymer waveguide elements. Details of device design and fabrication for distributed TWPD integrated with polymer optical waveguides were described in a previous report[13]. In this report, the separation between BCB polymer waveguides was designed to be 250 µm to match our dual output fiber coupler and the absorption layer thickness was reduced to 0.5 µm from 1 µm in order to improve the transit time limited bandwidth of metal-semiconductor-metal (MSM) photodetectors.

2. Balanced coherent detection

Figures 1(a) and 2(b) show schematic diagrams of balanced coherent detection and monolithic distributed balanced TWPDs integrated with BCB optical waveguides. The photocurrent from each detector can be written

where R is the responsivity of the photodetector, P_s, ω_s and φ_s are the power, frequency and phase of the signal, and P_LO, ω_LO and φ_LO are the power, frequency and phase of the LO, respectively. Since the LO power is much larger than the signal power and the photocurrent is proportional to the geometric mean of LO power and signal power, the SNR can be improved by increasing LO power. The balanced photocurrent is expressed by

 

Fig. 1. Schematic diagram of (a) balanced detection, and (b) distributed balanced TWPDs integrated with BCB optical waveguides.

Download Full Size | PDF

The intensity noise from the LO associated with the term P_LO is eliminated by subtracting the photocurrents in balanced photodetectors. For CW homodyne, that is ω_s=ω_LO, Eq. (3) reduces to

In direct detection, the intensity noise from the light source can be ignored because the optical power of the signal is not high enough in normal operation. However, it is not negligible in coherent detection because coherent detection uses higher optical power LO. At some point, even though P_LO increases to increase the received signal, the SNR cannot be improved any further. The importance of reducing RIN in balanced coherent detection under high power LO condition can be seen by considering the contribution of intensity noise in SNR. When RIN is considered, SNR can be written [1],

where σ_s, σ_T and σ_I are the shot noise, thermal noise and the intensity noise, respectively. Δf is the effective noise bandwidth, F_n is the noise figure, and R_L is the load resistor. In coherent detection, the photodetectors can reach the shot noise limit condition (σ²_s≫σ²_T) by increasing P_LO. Because σ²_I involves the square of P_LO, the contribution from σ_I becomes comparable to σ_s as P_LO increases. If P_LO is increased beyond this point, the SNR starts to decrease unless P_s is increased. Therefore, P_s should also be increased in order to maintain SNR; the increased amount of P_s is called a power penalty. As seen in Eq. (3), the RIN can be reduced by using balanced detection.

3. Experiments

3.1 Epitaxial layer and fabrication

The devices used in this report were designed to have 250 µm separation between BCB polymer waveguides in order to match our dual output fiber coupler. In addition the absorption layer thickness was chosen to be 0.5 µm in order to achieve higher transit time limited bandwidth of MSM photodetectors. In our previous report[13], the separation and the absorption layer thickness were 70 µm and 1 µm, respectively. The epitaxial layer structure for active photodetectors consists of a 20 nm In_0.52Al_0.48As diffusion barrier layer, a 0.5 µm In_0.53Ga_0.47As absorption layer followed by a 20 nm In_0.53(Ga_xAl_1-x)0.47As graded layer and finally a 40 nm In_0.52Al_0.48As Schottky barrier enhancement layer (SEL) on top. According to the simulation, the transit time limited bandwidth of the MSM photodetector can be improved 60 % by reducing the absorption layer from 1 µm to 0.5 µm when finger spacing and width are 0.5 µm.

The fabrication procedure begins with a 3.3 µm high mesa formation for the active photodetector regions by wet etch and reactive ion etching (RIE) using Si₃N₄ as an etching mask. After removing the Si3N4 mask using buffered HF, a 3.7 µm thick SiO₂ was deposited for the cladding layer and then a planarization process by RIE was performed to remove SiO2 deposited on the mesas for the photodetectors. After opening the detection areas for the interdigitated finger electrodes, the finger electrodes (Ti/Au=100Å/2300Å) and coplanar waveguide (CPW) transmission lines (TLs) of Ti/Au=200Å/10000Å were formed by lift-off. The BCB polymer was spin coated and cured for 1 hour at 250 °C. After photolithography, 3 µm wide, 3.4 µm high ridge BCB optical waveguides were formed by RIE.

3.2 Measurement results and discussion

The 3 dB bandwidth of the distributed TWPD was measured by coupling light into one channel of the waveguide pair at a time. A 1.55 µm DFB laser operating in CW mode was modulated by a 20 GHz Mach-Zehnder modulator driven by a microwave signal from the HP 8753C network analyzer, and the intensity modulated light was coupled into each side of the polymer optical waveguide pair. Figures 2(a) and 2(b) show the measured frequency responses of two distributed TWPDs : one from 5 MSMs and the other from 8 MSMs. Each MSM has 0.5 µm width, 0.5 µm spacing finger electrodes and 0.5 µm thick InGaAs absorption layer. The measured dark currents were 16 nA to 33 nA at 5 V. From DC photoresponse measurement, the responsivity of distributed TWPDs having 5 MSMs was estimated to be 0.35 A/W by assuming 35 % coupling efficiency from lensed fiber to BCB optical waveguide and ignoring loss due to roughness or reflection at the facet of BCB waveguides. The estimation of coupling efficiency was based on the photocurrent measurement by scanning the lensed fiber along horizontal and vertical directions. A beam intensity profile from lensed fiber was obtained from the measured photocurrent and 35 % coupling efficiency was estimated by considering the area of BCB waveguide facet. A 3 dB bandwidth of 20 GHz was measured at 7 V in both distributed TWPDs. The transit time limited bandwidth was calculated to be 20 GHz for MSMs having 0.5 µm finger width and spacing and 0.5 µm thick absorption layer.

 

Fig. 2. Frequency responses of distributed traveling-wave photodetectors at 7 V bias. Each MSM has 0.5 µm finger electrode widths and spacings. (a) both channels of 5 MSM PDs and (b) 8 MSM PDs. Dots indicate measured data and the solid lines are fitted lines.

Download Full Size | PDF

 

Fig. 3. Measurement set up for balanced detection.

Download Full Size | PDF

The measurement results agreed well with the estimated bandwidths. Since same bandwidths up to 20 GHz were measured in both distributed TWPDs, we can see that the bandwidths of both devices are still limited by transit time, not by velocity mismatch. The velocity mismatch limited bandwidths were estimated to be 58 GHz for 5 MSM PDs and 33 GHz for 8 MSM PDs in distributed TWPDs.

The measurement setup for balanced coherent detection is shown in Fig. 3. A 1.5 µm DFB laser operating in CW mode was divided by 50 : 50 in a power divider. One channel (Mod channel) was connected to a Mach-Zehnder modulator for intensity modulation and the other channel (LO channel) was amplified by EDFA without modulation. Then both channels were combined in a 3 dB coupler and connected to a dual output coupler. The dual-output coupler was coupled to the polymer optical waveguides of a distributed balanced TWPD. The dual output coupler consists of two single mode flat end fibers (9 µm core) mounted on V-grooved glass and the fibers were separated by 250 µm. Since the fibers in the dual output coupler are flat ended, the maximum coupling efficiency from fiber to BCB waveguide is estimated to be about 15 % without including losses due to reflection and roughness at the facets of the BCB waveguides. To provide biases to ground lines of the CPW, a probe that puts capacitors between signal and ground probes was used for isolation of the RF signal from DC bias [14]. One device used in the demonstration of coherent detection had 5 distributed MSMs in each side of the polymer optical waveguide pair. The MSMs had 0.5 µm width and 0.5 µm spacing finger electrodes. The measurement result for balanced detection is shown in Fig. 4. The average optical power of the signal was estimated to be 4 µW by assuming 15 % coupling efficiency. The center frequency of RF modulation was 15 GHz. First, the response of a single distributed TWPD measured by blocking the LO channel at V₁=5 V was -82 dBm (single channel without LO). Second, the response of a single distributed TWPD was measured after the LO channel was connected (unbalanced) at V₁=5 V and a response of -62 dBm was obtained. The signal was enhanced by 20 dB due to the strong LO signal in unbalanced coherent detection. LO power of 2.1 mW (3.2 dBm) was provided through each channel. Finally, a balanced photoresponse was measured at V₁=5V and at V₂=-5 V, and an output power of -56 dBm was measured. The signal was enhanced by 6 dB because the power generated from both channels was used in balanced detection. The photocurrents in each channel were monitored and the measured photocurrents were I1=16 µA and I₂=-15 µA. Common mode rejection ratio (CMRR) is a measure indicating how well the photodetectors are balanced to reduce the common mode noise. CMRR is defined as 20 log (I _COM/I_DIFF), where I _COM is total photocurrent and I _DIFF is the photocurrent difference of the two sides of the distributed TWPDs [4]. The CMRR was 30 dB under the above measurement condition for balanced detection, which is good indication that our TWPDs and other fiber elements, including the 3 dB fiber coupler and the dual output coupler, are well balanced.

 

Fig. 4. Signal enhancement in balanced detection. Spectra measured at a resolution bandwidth of 300 kHz, and video bandwidth of 300 Hz.

Download Full Size | PDF

 

Fig. 5. Signal output power gain dependence on LO power at 15 GHz. Square dots indicate measured data and the dotted line is ideal linear response.

Download Full Size | PDF

Figure 5 shows the signal output power gain as a function of provided LO power in balanced detection. The signal output gain was defined as the ratio of the output power in balanced TWPDs with LO to the output power of single channel TWPDs without LO. Up to 6.1 dBm LO power, the output power increased linearly, but output power decreased at the LO power of 7.3 dBm.

4. Conclusions

We have demonstrated coherent detection using balanced distributed TWPDs monolithically integrated with single mode polymer optical waveguides. The balanced distributed TWPDs used for the demonstration had 0.5 µm finger electrode width and spacing and 0.5 µm thick InGaAs absorption layer, and transit time limited 3 dB bandwidths of 20 GHz were measured. The signal enhancement was measured up to 26 dB at a modulation frequency of 15 GHz using a balanced coherent detection scheme. The demonstration shows that using polymer material as optical waveguides can provide an effective alternative to semiconductor materials for future photonic integrated circuits because polymer waveguides can be laid out easily on semiconductor active optical components. For further on-chip integration, integrating a 3 dB coupler with our TWPDs can be done without much increased complexity.