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Abstract
Femtosecond lasers have been widely employed in scientific and industrial applications, including the study of material properties, fabrication of structures on the sub-micrometer scale, surgical and medical treatment, etc. In these applications, the ultrafast laser is implemented either in free space or via an optical fiber-based channel. To investigate the light-matter interaction on a chip-based dimension, laser pulses with extremely high peak power need to be injected into an integrated optical waveguide. This requires the waveguide to be transparent and linear at this power, but also capable of providing a highly efficient and reliable interface for fiber-chip coupling. Contrary to the common belief that polymer materials may suffer from stability issues, we show that a polymer waveguide fabricated under simple and low-cost technology using only commercial materials can indeed transmit femtosecond laser pulses with similar characteristics as low-power continuous-wave laser. The coupling efficiency with a lensed fiber is ∼76% per facet. The pulse broadening effect in the polymer waveguide is also well fitted by the material and waveguide dispersion without nonlinear behavior. This study paves the way for developing a low-cost, highly efficient, polymer-based waveguide platform for the investigation of ultrafast phenomena on a chip.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Ultrafast optical techniques have been viewed as a powerful tool in laboratory. Thanks to the extremely high peak powers achieved by the pulses with widths on the pico/femtosecond scale, the strong light-matter interaction can trigger various physical, chemical, and biological reactions. Enabled applications include imaging [1], sensing [2], optical communication [3,4], micro/nanofabrication [5–7], integrated optical measurement [8,9], manipulation of biological systems [10], etc. To implement the ultrafast light pulses, delicate and expensive optical systems must be built to guide, deflect, and focus the light, requiring all optical elements to be stable under the high peak power. Most of these systems are developed using free-space optics [11,12].
Optical fibers have also been adopted to transmit ultrafast laser pulses as the guided-wave channel can define the light path more flexibly and safely to the target, but also to collect the modulated pulses for analysis. Due to dispersion, polarization, and the nonlinear effect, the pulse shape will undergo distortion while propagating in a fiber, subject to the fiber type, length, and the environment. These distortions must be compensated, calibrated, and referenced to reveal the true information of the target when the laser pulse is used for material studies. However, the fibers are often meters long and it is challenging and costly to stabilize the fiber paths from mechanical vibrations, stress shift, temperature drifting, etc., so that the pulse shape stays constant during the investigation.
On the other hand, photonic integration circuit (PIC) technology using planar waveguides on a chip has matured over the years with applications spinning beyond communication [13] to sensing [14] and medical diagnosis [15,16]. The photonic waveguide chip is usually on the millimeter to centimeter scale, allowing a much easier technique to be applied to shield it from environment (packaging) and stabilize the waveguide condition. There have been studies of 2D materials integrated on a waveguide platform [17–21]. Some of them have adopted femtosecond lasers to examine the light-matter interactions in the guided wave domain [22–24]. However, the laser pulse transmission in the bare waveguide itself is not investigated thoroughly, leaving it difficult to interpret the effect of 2D materials from the data precisely.
Recently, polymer-based PIC technology has attracted much attention due to its advantages such as low-cost, simple technology for fabrication, low optical loss at customized wavelength windows, and high compatibility to integrate components from other material platforms [25–27]. However, the ability to transmit laser pulses of high peak power in a confined polymer waveguide has not been investigated thoroughly. Whether such a low-cost waveguide can withstand the transmission of periodic femtosecond laser pulses without damaging its chemical composition remains an open question. Moreover, the transmission of the femtosecond laser in a planar waveguide has not been well characterized to pinpoint its propagation loss (in dB/cm) and coupling loss with the fiber, nor have the pulse temporal characteristics been studied in detail.
In this work, we aim to characterize the transmission properties of ultrafast pulsed laser in polymer waveguides quantitatively. These waveguides are fabricated using commercial materials under mature technology and have been developed into commercial devices [28–30]. Femtosecond pulsed laser at 1550 nm with a peak power of around 20 kW and repetition rate of 100 MHz (average power 105 mW) is coupled into and out of the polymer waveguide via a pair of lensed fibers. Three lengths of the waveguide under the same design are measured. The cutback method is applied to find the propagation loss and coupling loss with the fiber. The results are comparable with the transmission characteristics using a low-power continuous wave (CW) laser at 1550 nm with a power of 1 mW. The pulse width broadening effect in the polymer waveguide is also reasonably verified by material and waveguide dispersion without nonlinear effect. We believe this work will prove essential in developing chip-based devices where ultrafast lasers in guided wave form are needed, e.g., for applications in material characterization, biological process control, chemical reaction triggering, etc.
2. Waveguide design and fabrication
Figure 1 shows the schematic view of the polymer waveguide. The cross-section is designed to be 3 µm × 3 µm. The core and cladding materials are taken from the ZPU12-RI series (ChemOptics) with a refractive index of 1.48 and 1.45, respectively. This series of optical polymer resins based on perfluorinated acrylate have greatly reduced the overtone absorption at near-infrared (NIR) wavelengths by fluorine substitutions of hydrogen atoms. The modification of molecular structures leads to improvement in optical loss, thermal stability and power capacity [29,31]. Similar waveguides have been used to develop dual-layer arrayed waveguide gratings (AWG) in our previous work [32]. The waveguide is single mode at 1550 nm and the calculated eigenmode profile is shown in the inset of Fig. 1. Thanks to the square-core design and the low intrinsic material birefringence, the TE and TM modes can be considered degenerate in terms of mode profiles, propagation loss, and fiber coupling loss. In this work, we have adopted a pair of lensed fibers (OZ OPTICS LIMITED, TSMJ-X-1550-9/125-0.25-7-2.5-14-2-AR) as a noncontact method to inject and collect light.
Fig. 1. 3D schematic view of the polymer waveguide with lensed fibers for in and out coupling. The core size is 3 µm × 3 µm. The pulse width of the incident laser is 51 fs (Gaussian fit). Inset: normalized real part of the electric field of the TE ground mode.
The fabrication follows a standard process [33], as shown in Fig. 2(a). The optical layers are spin-coated and cured by UV irradiation and baking. The waveguides are defined by standard contact photolithography and patterned with a thin metal (Ti) mask through a lift-off process. The polymer core layer is etched by inductively coupled plasma (ICP) etching and the metal mask is then removed by wet chemicals. After the top cladding is coated and cured, the wafer is diced into chips for measurement. Figure 2(b) shows the top view of the fabricated channel waveguides. The cross-sectional view of the waveguide facet is displayed in Fig. 2(c).
Fig. 2. (a) Flowchart of the fabrication process. (b) Top view and (c) cross-sectional view of the polymer channel waveguides. Inset in (b) shows a zoomed-in view of the single waveguide core.
2. Measurement system and calibration
The diagram of the experimental setup is shown in Fig. 3(a). The CW laser (EXFO T100-S HP) and pulsed laser (MenloSystems C-Fiber) can be switched manually. To determine the polarization state of the incoming light, polarization controllers are equipped for both laser sources. A variable fiber optic attenuator (VOA) is added to regulate the pulsed laser power. The fiber-chip alignment is facilitated by a set of fine positioning stages and the three-axis piezoelectric controller can move the fiber at a precision of ±10 nm along X, Y, and Z directions. The measurement bench is shown in Fig. 3(b). Lensed fibers are adopted in this work as they provide a beam spot close to the waveguide mode size and can therefore improve the field overlap / coupling efficiency. The noncontact coupling method also avoids the index matching fluid, which may bring extra unstable factors under femtosecond lasers. Photos of the chip and the coupling position with the lensed fibers are shown in Figs. 3(c)–3(e).
Fig. 3. (a) Experimental setup of the femtosecond laser and the CW laser with the alignment/coupling system. PMF: polarization maintaining fiber; VOA: variable fiber optic attenuators; SMF: single mode fiber; PC: polarization controller; PD: photodetector; PM: power meter. CW laser: continuous-wave laser. Inset (1): laser pulses captured by an oscilloscope. Inset (2): transmitted power vs. VOA turns. (b) Photograph of the measurement setup. (c) Photograph of the chip and the fibers on alignment stages. (d) and (e) Microscope images showing the fiber-chip coupling.
An autocorrelator (APE PulseCheck 50) and a power meter (THORLABS PM100D) are introduced for synchronous and real-time monitoring of the pulse width and laser power. The output of the femtosecond laser is separated by a 1:99 output coupler where 1% of the power is transmitted to the power meter and the remaining 99% is detected by the autocorrelator for pulse width measurement. Additionally, an optical spectrum analyzer (Yokogawa AQ6370C) is equipped for spectrum analysis. For the CW laser measurement, the output is switched to an optical component tester (EXFO CT440-PDL) for detection.
Calibration is performed in the following steps to record the status but also to verify the stability of the measurement system. First, the femtosecond laser is fed directly to the power meter and the measured average power fluctuates in the range of 105.20-105.34 mW in 4000 seconds, showing a stable performance. The laser is then connected to a high-speed photodetector and subsequently to the oscilloscope. The repetition rate stands at 100 MHz, as shown in the inset (1) of Fig. 3(a). The energy of a single pulse is calculated to be 1.05 nJ. Next, the laser is connected to the autocorrelator, and the measured Gaussian profile shows a width of 51 fs, resulting in a peak power of 20.6 kW.
Secondly, the VOA is added after the femtosecond laser and its performance is recorded as shown in the inset (2) in Fig. 3(a). The position and number of full turns on the knob are marked to regulate the level of attenuation. The average power of the pulsed laser can be continuously varied from 0 to 92.8 mW.
Finally, we perform the fiber-to-fiber transmission measurement to label the status of the entire system. The fiber-to-fiber alignment is first adjusted coarsely with the help of a visible-light laser source and then switched to the CW laser (EXFO T100-S HP) for fine adjustment with the piezoelectric controllers. For the CW laser, the fiber-to-fiber transmitted power is measured as -7.69 dBm at 1550 nm wavelength with 1 mW input laser power. The 7.69 dB loss includes the intrinsic 6 dB loss of the CT440-PDL detector system with full polarization control and the fiber-free space-fiber loss. For the femtosecond laser, the fiber-to-fiber transmitted power is 57.11 mW, amounting to a loss of 2.11 dB. This loss is mainly attributed to the fiber-free space-fiber coupling loss. The pulse width increases to 264 fs because of the extra fibers included in the measurement.
3. Power transmission measurement
After the system calibration, the waveguide chip is placed between the lensed fiber pair and fine aligned, as shown in Figs. 3(d) and 3(e). The transmission measurement is normalized to the fiber-to-fiber transmission. In this way, the influence of the system is eliminated, and the intrinsic properties of the polymer waveguides can be studied.
For the CW laser transmission at the milliwatt level, the polymer waveguides already show good linear response and stability. Standard cutback measurement is performed using three sets of identical waveguides but with different lengths, i.e., 1 cm, 2.5 cm, and 3.5 cm waveguides. The results are shown in Fig. 4(a). The linear fitting reveals a propagation loss (slope) of 0.77 dB/cm and a coupling loss of 2.13 dB for both facets (intercept), i.e., 1.065 dB/facet. These results agree well with the standard technology [32,34].
Fig. 4. (a) Measured waveguide transmission under CW laser and pulsed laser for the 1-cm-long, 2.5-cm-long, and 3.5-cm-long waveguides. (b) Transmitted power and normalized transmission versus input average power of the femtosecond pulsed laser for the 1-cm-long, 2.5-cm-long, and 3.5-cm-long waveguides. Error bars indicate the standard deviations.
For the femtosecond laser, the linear response and the stability during transmission need to be verified. Figure 4(b) shows the transmitted power (in mW) and normalized transmission (in dB) versus the input average power of the femtosecond laser for the 1-cm, 2.5-cm, and 3.5-cm-long waveguides. At each waveguide length, the VOA is adjusted to 4, 4.25, 4.5, 4.75, 5, and 6 turns to regulate the average laser power, as indicated in inset (2) of Fig. 3(a). Each measurement lasts for approximately 20 minutes. The responses of the waveguide at all three lengths exhibit good linearity to the input power. Then, the transmission is normalized to the previously measured fiber-to-fiber result. The values are plotted in the bottom half of Fig. 4(b). Since non-contact fiber-waveguide-fiber coupling is implemented, the coupling states are more easily disturbed by environmental factors compared to the contact facet coupling with further assistance by the index matching fluid. Therefore, fluctuations in waveguide transmissions during the 20-min measurement are observed, but all stay within the ∼0.1 dB margin as indicated by the error bar (standard deviation). The results of all 6 average powers exhibit the stability of the waveguide within the total measurement time of 2 hours.
The cutback measurement under the femtosecond laser is also plotted in Fig. 4(a), in which the VOA is turned (6 turns) for minimal attenuation. The propagation loss and coupling loss for the pulsed laser transmission are found to be 0.70 dB/cm and 1.17 dB/facet, respectively. The results are very similar to those under a CW laser. Additionally, the transmission spectra under CW and pulsed laser are plotted in Figs. 5(a) and 5(b), respectively. The dip in the waveguide-transmitted spectrum at 1620 nm is attributed to the intrinsic absorption of the polymer material. The results confirm that the polymer waveguide is linear under both CW and femtosecond laser transmission, allowing us to study the pulse broadening effect without considering the nonlinear effect and without instability issues.
Fig. 5. (a) Measured power spectra in the 1500-1630 nm wavelength range with and without the 3.5-cm-long waveguide under CW laser. (b) Measured power spectra in the 1450-1650 nm wavelength range with and without the 3.5-cm-long waveguide under femtosecond laser.
4. Pulse broadening study
Apart from attenuation, the femtosecond pulses propagating in a waveguide undergo changes in the pulse shape due to optical nonlinearity, dispersion, and other effects [35–37]. The previous measurements have shown that the polymer waveguide has low propagation loss and ruled out the nonlinear effect. Therefore, we only consider dispersion in modelling the measured pulse broadening through the waveguides. As shown in Fig. 1, the polymer waveguide adopts a square core and the material birefringence is low, allowing us to further neglect the polarization mode dispersion. Thus, in this work, we attribute the pulse broadening only to chromatic dispersion, which comes from material and waveguide dispersion.
To analyze the pulse broadening effect quantitatively, we apply the cutback method again and measure the pulse width after propagating in a polymer waveguide of three different lengths, i.e., 1 cm, 2.5 cm, and 3.5 cm. The results are compared to the fiber-to-fiber measurements to extract the influence of the added waveguide. Figure 6(a) shows the measured autocorrelation traces of the femtosecond laser as system input, after fiber-to-fiber coupling, and further after propagating through a 3.5-cm-long waveguide. The increase in the pulse width, measured as the full width at half maximum (FWHM) of the Gaussian-fitted profiles, reveals a distinctive broadening effect after the waveguides are coupled between the lensed fiber pairs in the system.
Fig. 6. (a) Gaussian-fitted autocorrelation traces of transmitted femtosecond laser pulses. Orange curve: system input. Green curve: through aligned lensed fiber pair and single mode fibers in the system. Red curve: through an added 3.5-cm-long waveguide. (b) Measured pulse widths versus waveguide length. The solid line is the fitted relation using the average values. The error bars indicate the standard deviations of the measurement. The gray area indicates the approximated area for the fitted lines under measurement inaccuracy.
Next, we complete the cutback measurement by tracing the pulse widths after propagating through the 1-cm-long and 2.5-cm-long waveguides. For each length of the waveguide, multiple tests are performed within 30 minutes, during which the autocorrelator automatically collects the data and outputs an average value of the pulse widths using Gaussian fit. The results are plotted in Fig. 6(b). The hollow triangles represent the average values, and the error bars stand for standard deviations. The uncertainties may originate from the scattering centered at micro defects produced during wafer dicing on waveguide facets. The scattering sates may vary from time to time due to system disturbances, which results in uncertainties in the pulse width measurements. The linearly fitted solid line reveals the relation between the pulse width and waveguide length. The gray shadow indicates the approximated area for the fitted line considering the measurement inaccuracies at each data point.
The slope of the linear fitted lines in the gray region in Fig. 6(b) indicates an average pulse broadening coefficient of 7.58 fs/cm and a possible range of 5.80-9.35 fs/cm. The intercept of these lines gives a value ranging from 473.29 to 481.36 fs. It is surprising to see that the intercept is much larger than 264 fs as the case of fiber-to-fiber transmission without the waveguide. It means that a net increase of the pulse width exists even when the waveguide length reduces to zero. We believe this is caused by the coupling process between the fiber and the waveguide mode, i.e., the waveguide coupling by mode field overlap takes time and affects the pulse width. This process is commonly believed to be instantaneous but can be revealed in the femtosecond scale. We leave the systematic study on this newly discovered phenomenon to our future work and focus here on the analysis of the pulse broadening coefficient in the polymer waveguide. Meanwhile, the peak intensity in the 1-cm-long waveguide is calculated to be 1.08 × 1014 W/m2, considering the ∼484 fs pulse width and ∼76% coupling efficiency for a single facet. However, this peak intensity is still at least one order smaller than the needed intensity for the second harmonic generation (SHG) of typical waveguide material, e.g., Au in plasmonic waveguides for nonlinearity generation [38]. Therefore, though the χ(2) of the polymer material is currently unknown, nonlinearity excitations are speculated to be difficult regarding the existing peak intensity confined in the waveguide.
It is established that in the dispersive guided wave system, the effective refractive index n(ω) of the mode varies with the frequency/wavelength of the guided light, endowing different frequency components with different propagation constants β(ω). The effective refractive index theory also simultaneously unifies the joint effect of the material properties and the waveguide structure [39–41], in which the material dispersion is the dominating factor. Mathematically, the mode propagation constant β(ω) can be expanded into a Taylor series around the center angular frequency of the pulse spectrum as
Figure 7(a) shows the refractive indices of the polymer cladding and core material (ZPU450 and ZPU480) measured by an ellipsometer (Eoptics Technology SE-VM-L) in the 380-1700 nm wavelength range. The material data are imported into a commercial waveguide eigenmode solver and the waveguide with the same structure as shown in Fig. 1 is modeled. The effective index of the ground mode versus wavelength is plotted in Fig. 7(a). In the wavelength range of 1450-1650 nm, consistent with the range of the measured spectra, the group index and chromatic dispersion parameters are calculated using Eqs. (2),(3) and plotted in Fig. 7(b).
Fig. 7. (a) The dispersion curves for the polymer cladding and core material, both measured from an ellipsometer, as well as the simulated effective index neff of the fundamental mode in a commercial eigenmode solver. (b) Calculated group index and chromatic dispersion parameter D versus wavelength of the polymer channel waveguide.
At the central wavelength of 1550 nm, D is calculated to be around -64.80 ps/nm/km, which demonstrates the characteristics of normal dispersion. The absolute value of the pulse width variation can be expressed as
where Δλ is the 1/e spectral width of the transmitted pulse and L is the waveguide length. Δλ is measured as 10.71 nm from Fig. 5(b), and the pulse broadening coefficient is calculated to be 6.94 fs/cm, which falls within the scope as the measured average value of 7.58 fs/cm and range of 5.80-9.35 fs/cm. Our measurement and theoretical calculations demonstrate that femtosecond pulse broadening in the chosen polymer waveguides can be characterized accurately using the cutback method and subsequent analysis using a simple chromatic dispersion model.5. Conclusion
To summarize, we experimentally demonstrate the transmission characteristics of a femtosecond pulsed laser in a polymer channel waveguide. The femtosecond pulsed laser and the measurement system are first calibrated. The polymer waveguides show good linearity and stability under femtosecond laser transmission with an average power up to 92.8 mW. Cutback measurements using three waveguides of different lengths have shown virtually no difference under CW or femtosecond laser transmission, with propagation losses characterized as 0.77 dB/cm and 0.70 dB/cm, respectively.
Furthermore, the cutback measurement is also applied for the pulse width broadening study. Experiments indicate that the pulse broadening coefficient in the waveguide has an average value of 7.58 fs/cm and stays in the range of 5.80-9.35 fs/cm. A classic chromatic dispersion model is built considering only the material and waveguide dispersions. This simple model is used to calculate the pulse broadening coefficient in the chosen polymer waveguide, as 6.94 fs/cm. The result agrees well with the experiment.
Our work demonstrates that femtosecond pulses can indeed transmit in polymer waveguides with stable and predictable time-domain properties, making the polymer waveguide platform an attractive candidate for the study of ultrafast phenomena on the chip scale. This work also serves as a foundation for the quantitative investigation of the light-matter interaction using novel materials and structures, integrated on a low-cost, compact, and flexible platform beyond free space and fiber channels.
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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