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Abstract
We demonstrate a monolithically integrated photonic integrated circuit (PIC) for terahertz spectroscopy with wide spectral bandwidth. The PIC includes two widely tunable sampled grating DBR (SG DBR) lasers, semiconductor optical amplifiers (SOAs), and passive components to combine signals. The SG DBR lasers cover 22 nm and 24 nm tuning range, respectively, with 4 nm overlap in the C band. The side mode suppression ratio (SMSR) exceeds 37 dB with a linewidth below 4.3 MHz. We used the PIC to generate THz radiation with a state-of-the-art photodiode emitter. The measured THz power spectrum between 0.03 and 1 THz compares well with the spectrum generated with commercial tunable laser sources. This demonstrates the suitability of our PIC for future miniaturized continuous wave (cw) THz systems.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Terahertz (THz) technology is receiving much interest in a large variety of different fields [1,2] like e.g. bio and medical applications [3,4], spectroscopy [5,6], and also for communications [7]. For these different applications, many approaches have been studied so far [8]. For a continuous-wave (CW) THz source based on photonics technology, mainly two methods are widely investigated: a multi-wavelength source and optical heterodyning, respectively [7,8]. Using a multi-wavelength source, represented by a mode-locked laser with an external modulator or optical filter, a good spectral purity can be achieved, however the THz bandwidth is limited. The optical heterodyning method using two different optical sources has the main advantage of a large THz bandwidth at the price of a rather poor frequency stability [9]. Traditional approaches to realize THz sources use discrete optical components. Such an optical system comprising a bunch of different packaged components is large in size, and hence the system reliability is limited.
A photonic integrated circuit is a promising solution for a THz system in order to significantly reduce cost and size, and to achieve high reliability. All required optical components such as laser sources, waveguides, couplers, splitters, modulators and amplifiers, can be integrated in such a PIC. The PIC size is in the range of several mm only, which is much smaller than a traditional system composed of different packed single elements. Moreover, PICs can be fabricated in large numbers such that scaling up for high volume and low costs is possible. Today, open-access generic foundry platforms [10,11] offer all the required integrated components, i.e. building blocks, to fabricate the optical engine of a broadband CW THz system.
Hitherto, such PICs for THz applications have been realized with tunable distributed feedback (DFB) lasers and distributed Bragg reflector (DBR) lasers mostly, because of their technical maturity [9,12–17]. However, the narrow tuning range of DFB and DBR lasers limits the generated THz bandwidth. For example, usual DFB lasers provide a few nm tuning range only. DBR lasers with a separately tuned passive DBR section allow for larger tuning of about 15 nm [18]. PICs with two DFB lasers combined by a multimode interference (MMI) coupler [12] demonstrated a THz bandwidth of 1.25 THz by thermal tuning of the DFB lasers. References [9,13,14] have a similar design with [12] but include additional high-speed photodiodes (PD). Reference [15] coupled four DFB lasers in a PIC to cover a broad tuning range, providing 2.25 THz bandwidth in total. A DFB laser with an integrated tunable DBR was reported in [16] and enabled THz emission up to 1.5 THz. Using a hybrid integration approach based on a polymer PIC combined with InP chips [17], a THz bandwidth of 4.2 THz could be demonstrated. The good thermal properties of a polymer DBR allowed for lasers with a tuning range of 20 nm. However, the PIC requires additional hybrid integration of an InP based gain chip. Si-photonic PICs using ring lasers with 42 nm tunable laser have also been demonstrated [19], but also in this case the hybrid integration of an InP based gain chip is required.
In this paper, we propose a novel fully integrated PIC as an optical source with high tuning bandwidth. It consists of semiconductor optical amplifiers (SOAs), waveguides, MMIs, thermal optic phase modulators (TOPMs), photodetectors (PDs), spot size converters (SSCs), and widely tunable SG DBR lasers. For PIC fabrication, we used the generic InP foundry platform developed at Fraunhofer HHI, in which customers can design and fabricate their own PICs within multi-project wafer runs [10]. We designed the PIC for a coherent THz spectrometer [20,21], which requires two output ports with independent phase modulations for each wavelength. In this paper, we investigated the THz power generated with the help of the tunable laser PIC in order to prove its functionality without using TOPMs. Table 1 organized key performance of the proposed THz PIC.
Table 1. Key performances of the THz PIC
2. Design and simulation
Figure 1 shows a microscopic image of the realized PIC. The PIC includes two SG DBR lasers with different tuning areas, namely laser 1 and laser 2, consisting of the front SG DBR, gain, phase, and rear SG DBR sections. The lengths of the sections are 500 µm, 400 µm, 120 µm, and 800 µm, respectively. The lasers are based on a ridge waveguide with a Fe-doped InP substrate. The gain section includes InGaAsP multi-quantum wells (MQWs) on top of a layer with photoluminescence peak at 1.3 µm (Q1.3), and the other sections are designed by selectively etching the MQWs layers. Using direct-writing electron-beam lithography the SG DBR was patterned into the Q1.3 layer. The coupling coefficient was set to 120 cm-1. Details of the layer information of these lasers are listed in Ref. [22]. SOAs have the same layer stack as the laser gain sections. The passive building blocks, such as MMI couplers, TOPMs, and SSCs, are defined on an InGaAsP layer with a bandgap of 1060 nm (Q1.06). Butt-joint sections connect the ridge waveguide from the lasers and SOAs to the deeply etched passive waveguides with a tapered structure for optimal coupling.
Fig. 1. (a) Microscope image and (b) schematic image of the THz PIC. PDs are only absorbers to reduce back reflections and simplify output ports.
MMI2 and MMI3 divide the emitted light from laser 1 and 2, and MMI1 and MMI4 combine both wavelengths. Each unused port of MMI2 and MMI3 is connected to PD1 and PD2, respectively, to prevent reflection when the port is abruptly ended. Such an end of waveguide can generate reflection from SOA’s amplified spontaneous emission (ASE) or any back reflection in the PIC. To simplify the output, we connect one of the output ports from MMI1 and MMI4 to PD1 and PD2, respectively. The waveguide structure between MMI1 and MMI3, or MMI2 and MMI4 shows a ring cavity, but it includes PD. Hence, there is no effect of the ring structure. Output ports from MMI1 and MMI4 are coupled to SOAs. A SSC with an angle of 7° couples the amplified light into an optical fiber. An anti-reflection coating of the facet minimizes back reflection from the PIC to air interface. TOPMs are designed to control the phase of the light from laser 1 and laser 2, but not used in this report. Predicted losses from the passive building blocks are as follow: MMI: 3.5 dB, TOPM: 1 dB, SSC for fiber coupling: 3 dB and butt-joint : 2 dB.
We designed the SG DBR lasers to cover different wavelength ranges with a small overlap. The design parameters of the SG DBRs are optimized based on the Vernier effect, which selects emission wavelengths from two slightly different comb-like reflections [23]. The SG DBR consists of periodic structures, including waveguides and gratings repetitively. The length of the periodicity (Z0) decides the peak spacing of the comb-like reflection (Δλ). The Δλ of the front and rear SG DBR (ΔλF, ΔλR) predominantly determines the tuning range. Furthermore, the length of the gratings (Z1) is related to the envelope of the reflection spectrum. All the design parameters for the SG DBR are explained in Ref. [22,24]. Both laser 1 and laser 2 have a target tuning range of 27 nm with a different center wavelength of 1538 nm and 1562 nm, respectively. Together, the two lasers cover 1524.5 nm - 1575.5 nm with an overlap of 3 nm. Corresponding ΔλF and ΔλR for the target tuning range are 3.5 nm and 3.1 nm, and the expected maximum reflections of the front and rear SG DBR are 25% and 70%. Table 2 shows details of the design parameters. NF and NR denote the number of bursts in the front and rear SG DBR. Fig. 2 (a) and (b) show simulated reflection spectra of laser 1 and laser 2 calculated with the transfer matrix method (TMM), based on the design parameters in Table 2. Figure 2 (c) and (d) depict multiplications of the reflections in Fig. 2 (a) and (b). SMSR is related to a difference between the central peak and the second strongest peak in Fig. 2 (a) and (b), which shows about half. The experimental result of SMSR was demonstrated over 37 dB, and we will discuss it in the next section.
Fig. 2. TMM simulation of laser 1 and 2. (a), (b) Reflection spectrum of front and rear SG DBRs. (c), (d) Multiplication of the reflections of the front and rear reflections in (a) and (b).
Table 2. Detailed parameters of the SG DBR lasers
3. Experimental measurement
The realized PIC was mounted on an Au-coated heat sink and stabilized with a thermoelectric cooler (TEC) at 20°C to test the optical performance. Contact pads on the top of the PIC were connected to the control sections in the PIC. We contacted a multi-needle probe to the pads for applying currents. Figure 3 (a) and (b) illustrate pulsed PI curves with an integrating sphere from SSC1. The current applied to SOA1 was 60 mA, and there was no applied current on the tuning sections. The PI curves show about 3 mW facet power at 100 mA gain current with 2.2 V. Threshold currents of the lasers are 41 mA and 43 mA for laser 1 and laser 2, respectively. The total power of two working lasers is expected the sum of each laser output. Furthermore, different SOA currents affects the optical output power, which was modeled and experimented as [25]. We used 60 mA to demonstrate over a 10 dB optical amplification. Laser 1 shows slightly lower power than laser 2 due to a curved waveguide with 200 µm-radius from MMI3 to MMI1, which has a larger loss for coupling to SSC1. In this case, the light from laser 2 passed through straight waveguides to SSC1, including lower loss than curved waveguide, therefore power of laser 2 is larger than laser 1. Moreover, laser 1 is closer to the SOA than laser 2 as in Fig. 1, such that the higher temperature of the SOA affects the thermal roll-off of laser 1.
Fig. 3. Pulsed PI measurement of laser 1 and 2 with 6 mA SOA current (a) PI curve. (b) Voltage measurement
Figure 4 (a) and (b) depict peak wavelengths of laser 1 and 2 with different tuning currents, namely a tuning map. An optical butt fiber was used to extract the light from SSC1 of the PIC and feed it into an optical spectrum analyzer (OSA). We applied 100 mA to the gain section, and 60 mA to SOA1. Front and rear currents were used between 0 and 45 mA and from 0 and 35 mA, respectively. Figure 5 (a) and (b) illustrate the selected optical spectra from Fig. 4 (a) and (b) and the corresponding SMSRs. From Fig. 4 (a) one obtains, 24 nm tuning for laser 1 (1526 nm – 1550 nm), and 22 nm tuning for laser 2 (1547 nm - 1569 nm). Laser 2 has a smaller tuning range than laser 1 because of its lower gain at longer wavelengths. Note that the overlap between the two lasers is 4 nm. The SMSRs are depicted in Fig. 5 (b). The lowest SMSR is 37 dB whereas the maximum amounts to 48 dB. Hence, the SMSR of this PIC is > 37 dB for all wavelength. Figure 4 and 5 show that lasers can access any wavelength in the tuning range with higher SMSR. The laser power varies with wavelengths as shown in Fig. 5 (a), but optimized currents on the gain section can improve the power distribution. We also want to point out that the tuning rage of this PIC, which amount to 43 nm, would allow for generating THz signals with a bandwidth of 5.36 THz. To the best of our knowledge this is a record value for monolithically integrated PICs.
Fig. 5. (a) Optical spectrum of laser 1 and laser 2 measured with an OSA. (b) SMSRs of laser 1 and laser 2.
In photonic cw THz systems the linewidth of the lasers determines the linewidth of the generated THz signal. In order to measure the optical linewidth of our laser PIC, we used a delayed self-homodyne method with 11 km-long single-mode fiber. An electrical spectrum analyzer (ESA) and a high-speed photodiode were used for the measurement. In Fig. 6 (a) the linewidth of laser 1 (blue) and laser 2 (orange) are shown. The linewidth was determined as the full-width half maximum (FWHM) of a Lorentzian fit to the measured data. This is exemplarily shown in Fig. 6 (b) for 1530.8 nm (laser 1) and 1554.9 nm (laser 2). Note that the measured linewidth ranges from 2.69 MHz to 4.07 MHz for laser 1 and from 3.02 MHz to 4.26 MHz for laser 2. Laser 2 has slightly larger linewidth than laser 1 because of its lower gain in the longer wavelength range. Less gain leads to more spontaneous emission, i.e., larger threshold current, which means that the contribution of spontaneous emission to the phase noise is higher [26]. The linewidth value shows lower in the center of the tuning range, due to the reflection spectrum is maximum at the center region as shown in Fig. 2.
Fig. 6. (a) Measured linewidth of laser 1 and laser 2 from Lorentzian fitting. (b) ESA spectrum of laser 1 and laser 2 with 1530.8 nm and 1554.9 nm, respectively. FWHMs of the Lorentzian fitting are 3.08 MHz and 3.17 MHz for laser 1 and laser 2.
The expected linewidth of the generated THz signal is still below 6 MHz ($\sqrt 2 \cdot 4.26$ MHz [27]), which provides high frequency resolution for a cw spectrometer [28]. Typically, ASE of the SOA degrades the optical linewidth. Such ASE increases the side lobes in the optical spectrum, of SG DBR laser, resulting in broaden linewidth. With an additional filter, e.g., Mach-Zehnder interferometer [29], we expect that the linewidth can be improved.
In order to demonstrate THz generation with this PIC, we performed spectral power measurements with a THz photodiode (THz-PD, PCA-FD-1550-100-TX-3 from TOPTICA Photonics) similar to [29]. A schematic of our setup is shown in Fig. 7. The optical beat signal of the PIC was connected to an isolator, preventing back-reflections from the measurement setup. An Erbium-doped fiber amplifier (EDFA) boosted the signal in order to drive the emitter with 8 mA photocurrent. The THz emitter was a state-of-the-art THz-PD with integrated broadband antenna, which was mounted on a silicon lens to radiate into free-space [30]. A 1% tap coupler allowed to monitor the optical beat signal with an OSA. The THz-PD converted the optical beat signal to an electrical THz signal and radiated into ambient air. In order to measure the emitted terahertz signal with a pyroelectric power detector (THz20 from SLT), a waveform generator (model 3390 from Keithley) applied a square wave bias voltage between 0 V and -1.5 V with a frequency of 20 Hz to the THz-PD. A transimpedance amplifier (TIA), which was calibrated together with the power detector, boosted the detector current. A lock-in amplifier (LIA, model 7265 from Signal Recovery) synchronized the detector signal with the bias of the emitter.
Fig. 7. Schematic image of the THz measurement setup. Blue and green lines represent optical and electrical signals, respectively.
Two SG DBR lasers were controlled to generate THz signal. The wavelength tuning for both lasers was achieved by controlling the SG DBR sections in analogy to the previous measurements to generate THz signals. We set the gain currents to 100 mA and adjusted it in the range ±10 mA in order to keep the peak power difference smaller than 3 dB. Furthermore, the applied current at SOA1 was 100 mA. Figure 8 (a) shows a representative optical spectrum with laser 1 and 2 operating simultaneously. The peak wavelengths are 1553.9 and 1555.5 nm for laser 1 and 2, respectively. The corresponding THz frequency is 199.6 GHz. Two commercially available tunable laser sources (TLS, i.e. AP3350A from APEX Technologies) were used instead of the PIC in the same measurement system to verify the PIC. Figure 8 (b) illustrates an optical spectrum of the TLS, with a difference frequency of 503.9 GHz (1552.9 and 1557 nm).
Fig. 8. (a) Optical spectrum of the PIC while laser 1 and 2 were operated for 199.6 GHz simultaneously. (b) Optical spectrum of two discrete tunable laser sources (TLS) under the same conditions as in (a) for 503.9 GHz. (c) THz power from wavelength tuning of laser 1 and 2 generated by a PIN-PD emitter. The PIC (blue) and the TLS (red) were used as laser sources, respectively.
The experimental measurement of the THz power is shown in Fig. 8 (c). We measured the THz power from 0.03 to 1 THz with the maximum value of 320 µW at 0.1 THz, and 1.8 µW around 1 THz. Due to the sensitivity of the pyroelectric detector, the measurements were limited to 1 THz bandwidth. However, the THz power spectrum measured with the PIC matches the measurement with the TLSs, which verifies that the PIC can potentially replace the commercial laser sources. Since the PIC can generate optical beat signals higher than 5 THz, we expect more broadband THz spectra from operation in a coherent, i.e. more sensitive, measurement system [20,21]. Since such a systems requires complex and dynamic operation of the PIC, the coherent THz system is still under investigation and will be presented elsewhere.
4. Conclusion
We propose a novel fully integrated tunable laser source for CW THz generation, which includes two tunable SG DBR lasers and passive components. The two SG DBR lasers offer a tuning range of 22 nm and 24 nm with 4 nm overlap. The combined tuning range of 43 nm corresponds to a bandwidth of 5.36 THz, which is the highest bandwidth obtained with any integrated tunable laser source. The SMSR is higher than 37 dB for all wavelength. The linewidth of the lasers is below 4.26 MHz, which would translate into a linewidth < 6 MHz of the corresponding THz signal. We performed THz power measurements between 0.03 THz and 1 THz and verified that the spectrum generated with the PIC compares well with the spectrum generated with commercial laser sources. Hence, this PIC can be an important building block of compact and cost efficient cw THz systems. The PIC was fabricated on the generic InP foundry platform of Fraunhofer HHI, has a small footprint and is ready to be produced in volume at low cost.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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