1. Introduction

Recently, the ultrafast properties of various optical devices, particularly those producing ultrafast light pulse trains, have been studied for high-speed optical communication and photonic network systems. The observation of these ultrafast light pulse trains is important to fully understand the properties of such devices. The need to evaluate various optical and photonic devices for ultrafast applications is expected to increase in future. However, there have only been a few studies investigating propagating light pulse trains from optical devices. One example of the techniques used in these studies is femtosecond time-resolved optical polarigraphy (FTOP) [1–5], a technique using a photon scanning tunneling microscope (PSTM) [6–8]. In FTOP, the pulses are indirectly observed by way of a nonlinear optical phenomenon. The PSTM allows observation of evanescent wave of the pulses by near-field optical microscopy. Since these techniques require repeating femtosecond pulses to acquire a motion picture of the propagating femtosecond light pulses, they are not always able to correctly observe the pulses when the repetition characteristics of pulses are not uniform. Furthermore, these techniques cannot obtain a motion picture of the propagating light pulses that is both spatially and temporally continuous, and they require precise alignment when constructing the optical system, such as fine adjustment of the optical path length. Owing to these disadvantages, frameless motion-picture acquisition of femtosecond pulse trains propagating in free space has not yet been successfully reported. One powerful technique capable of overcoming these problems is light-in-flight holography, which can observe propagating ultrashort light pulses as a spatially and temporally continuous motion picture at a desired speed [9–11]. This technique requires neither a nonlinear optical phenomenon nor repeating femtosecond pulses in principle.

To demonstrate the observation of a femtosecond light pulse train output from an optical device and propagating in air, which is useful for ultrafast applications, we present the observation results of the pulse train generated by an optical device that we call an integrated array illuminator. The observation results are obtained in the form of a spatially and temporally continuous motion picture acquired by using light-in-flight holography. We also discuss the reconstructed image of the femtosecond pulse train.

2. Integrated array illuminator

The integrated array illuminator, which we have been developing, is an effective optical device to generate a light pulse train from a single input light pulse [12–15]. A schematic diagram of the device and the principle of pulse train generation are shown in Fig. 1. The device consists of a diffraction grating, a glass substrate, and a prism. An incident light pulse is introduced to the substrate through the prism as an input light pulse. The input light pulse is totally internally reflected in the glass substrate. When the totally internally reflected light reaches the diffraction grating, a part of the energy of the beam propagating inside the glass substrate escapes as diffracted light to serve as an output light pulse. By repeating the total internal reflection in the glass substrate, an output light pulse train with both spatially and temporally equal intervals can be obtained. A light pulse train with uniform pulse intensities can be obtained by properly designing the groove depth of the grating [13,14]. The device is useful for high-speed optical communications and high-speed access to optical memories. Inclined light pulses, that is, not perpendicular to the propagating direction, are emitted from both sides of the device. The repetition rate of the light pulses is determined by the thickness of the glass substrate.

 

Fig. 1. Integrated array illuminator using a grating coupler.

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3. Light-in-flight holography

The basic optical setup to record propagating light pulses using light-in-flight holography is shown in Fig. 2. A light pulse generated by an ultrashort pulsed laser is divided into two light pulses by a beam splitter. One light pulse is introduced to the recording material as a reference light pulse. The other is introduced, at a certain angle, to a diffuser, and it sweeps over the surface of the diffuser as the pulse propagates. The light pulse scattered by the diffuser becomes the object light pulse. This object light pulse contains information of the light pulse propagating on the diffuser. Only when both light pulses meet on the recording material are interference fringes generated and recorded as the hologram. The information of the light pulse propagating on the diffuser is recorded on the hologram in the form of interference fringes. Since both the light pulse illuminating the diffuser and the reference light pulse respectively sweep over the diffuser and the recording material in the recording process, the status of the propagating light pulse at each instant is recorded in different parts of the recording material. To obtain a spatially and temporally continuous motion picture of the propagating light pulse, the hologram is scanned by a CW laser beam along the same direction in which the reference light pulse propagated.

 

Fig. 2. Basic setup of the light-in-flight holography: BS, beam splitter; OB1, OB2, microscope objectives; L1, L2, lenses.

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4. Experiment and discussion

We conducted an experiment to record and observe the propagating femtosecond light pulse train generated by the integrated array illuminator in the form of a spatially and temporally continuous motion picture.

The thickness of the glass substrate and the period of the grating were 1.15 mm and 593 nm, respectively. A schematic diagram of the optical setup used to record the light pulse train is shown in Fig. 3. A mode-locked Ti:Sapphire laser (MIRA900-D, Coherent Inc.) was used as the femtosecond pulsed laser. The pulse width and the center wavelength of the femtosecond laser pulses were 130 fs and 720 nm, respectively. Agfa Holotest 8E75HD was used as the recording material because it is sensitive at this wavelength.

A light pulse from the femtosecond pulsed laser was divided into two light pulses by a beam splitter. One light pulse was collimated by a microscope objective and a collimator lens, and was introduced to the recording material at a certain inclination angle as a reference beam. The other pulse was introduced to the prism on the glass substrate of the integrated array illuminator at an incident angle of 11.4°. The produced light pulse train was reflected by a diffuse-reflection screen to generate the object light pulses. The screen placed in contact with the array illuminator acts in the same manner as the diffuser used in the basic optical set up shown in Fig. 2. The light intensity on the recording material and the exposure time were 120 µW and 1.3 s, respectively. The light pulses were arranged to be nearly parallel to the screen to obtain a temporally longer motion picture. To record the femtosecond light pulse train on the recording material, the optical path length of the object light pulse and that of the reference light pulse were adjusted so as to interfere with each other.

 

Fig. 3. Optical setup for recording the light pulse train generated by the integrated array illuminator: BS, beam splitter; OB, microscope objective; L, lens.

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A schematic diagram of the reconstructed image of the generated light pulse train is shown in Fig. 4. The wide shaded lines indicate the path of the CW beam from the Ti:Sapphire laser. Although it is not necessary for the observation of the pulse train, the path of the CW beam is also recorded so as to easily recognize the path of the propagating femtosecond light pulse train.

Time sequential photographs extracted from the spatially and temporally continuous motion picture of the reconstructed image are shown in Fig. 5. The wide white lines indicate the path of the CW beam from the Ti:Sapphire laser. The light pulses are shown by the bright bands on the path of the CW beam.

A short clip of the observed motion picture is shown in Fig. 6. Each light pulse of the train moves on the path of the CW beam, and they propagate in the direction perpendicular to the diffraction grating. The time and spatial intervals depend on the thickness of the glass substrate and the incident angle to the prism. The temporal and spatial intervals of the adjacent light pulse are obtained as 19.7 ps and 3.17 mm because the thickness of the glass substrate and the incident angle to the prism are 1.15mm and 11.4°, respectively. The light pulse train at the left side of the array illuminator propagates in the same direction as the reference light pulse. The train at the right side propagates in the opposite direction. It is interesting to note that a back-propagating femtosecond light pulse train, which is the -2 order diffracted light from the array illuminator [14], can be observed on the path of the input beam. Also the following interesting features of the reconstructed images were found.

- Inclination angles of the reconstructed pulses differ depending on the propagation direction.

- Propagating speeds of the reconstructed pulses in the horizontal direction differ depending on the propagation direction.

- Widths of the reconstructed pulses differ depending on the propagation direction.

To reconstruct the actual shape and speed of the light pulse train, these differences should be analyzed and the reconstructed image should be corrected based on the analysis results. Although such analysis and correction are important, they are difficult to tackle in a short period and remain to be further investigated in future.

 

Fig. 4. Schematic diagram of the reconstructed hologram image of the femtosecond light pulse train.

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Fig. 5. Sequential photographs extracted from the spatially and temporally continuous motion picture of the propagating femtosecond light pulse train. The size of the scale bar is 5 cm. The wide white lines indicate the path of the CW beam from the Ti:Sapphire laser. The light pulses are shown by bright bands on the path of CW beam.

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Fig. 6. Motion picture of the propagating femtosecond light pulse train generated by the integrated array illuminator. (Multimedia file, 1.39 MB.) The femtosecond light pulse train is moving on the white lines representing the CW beams. The duration of the entire motion picture corresponds to 160 ps. A back-propagating femtosecond light pulse train is observed on the path of the input beam of the array illuminator.

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5. Conclusion

We observed the propagating femtosecond light pulse train generated by the integrated array illuminator in the form of a spatially and temporally continuous motion picture acquired using light-in-flight holography. It has been clearly observed that each pulse propagates in air and its pulse front is tilted by the diffraction grating. Also a back-propagating femtosecond light pulse train, which is the -2 order diffracted light pulse from the grating of the array illuminator, is observed in the path of the input beam of the array illuminator; this pulse train is difficult to observe using conventional techniques. These results demonstrate the usefulness of light-in-flight holography for the evaluation and characterization of recently developed ultrafast photonic devices [16–18] used for ultrafast optics and photonic information systems. Ultrashort light pulse propagating in the ultrafast photonic devices to be investigated can be observed by the technique using enough weakly scattering media, which hardly disturb the property of the device, attached to the devices.