I.INTRODUCTION

   An optical modulator is a device which can be used for manipulating a property of light – often of an optical beam, e.g. a laser beam. Depending on which property of light is controlled, modulators are called intensity modulators, phase modulators, polarization modulators, spatial light modulators, etc. A wide range of optical modulators are used in very different application areas, such as in optical fiber communications, displays, for active Q switching or mode locking of lasers, and in optical metrology.^[1]

According to the properties of the material that are used to modulate the light beam, modulators are divided into two groups: absorptive modulators and refractive modulators. In absorptive modulators the absorption coefficient of the material is changed, in refractive modulators the refractive index of the material is changed. ^[2]

The absorption coefficient of the material in the modulator can be manipulated by the Franz-Keldysh effect, the Quantum-confined Stark effect, excitonic absorption, changes of Fermi level, or changes of free carrier concentration. Usually, if several such effects appear together, the modulator is called an electro-absorptive modulator.

Refractive modulators most often make use of an electro-optic effect. Some modulators utilize an acousto-optic effect or magneto-optic effect or take advantage of polarization changes in liquid crystals. The refractive modulators are named by the respective effect: i.e. electrooptic modulators, acousto-optic modulators etc. The effect of a refractive modulator of any of the types mentioned above is to change the phase of a light beam. The phase modulation can be converted into amplitude modulation using an interferometer or directional coupler. ^[2]

Separate case of modulators are spatial light modulators (SLMs). The role of SLM is modification two dimensional distribution of amplitude and/or phase of an optical wave. ^[2]

II.TYPES OF OPTICAL MODULATORS

1.Acousto-optic modulators

An acousto-optic modulator (AOM) is a device which can be used for controlling the power, frequency or spatial direction of a laser beam with an electrical drive signal. It is based on the acousto-optic effect, i.e. the modification of the refractive index by the oscillating mechanical pressure of a sound wave.

The key element of an AOM is a transparent crystal (or piece of glass) through which the light propagates. A piezoelectric transducer attached to the crystal is used to excite a sound wave with a frequency of the order of 100 MHz. Light can then experience Bragg diffraction at the traveling periodic refractive index grating generated by the sound wave; therefore, AOMs are sometimes called Bragg cells. The optical frequency of the scattered beam is increased or decreased by the frequency of the sound wave (depending on the propagation direction of the acoustic wave relative to the beam) and propagates in a slightly different direction. (The change in direction is smaller than shown in Figure 1, because the wavenumber of the sound wave is very small compared with that of the light beam.) The frequency and direction of the scattered beam can be controlled via the frequency of the sound wave, whereas the acoustic power is the control for the optical powers. For sufficiently high acoustic power, more than 50% of the optical power can be diffracted – in extreme cases, even more than 95%.^[2]

Figure 1: Schematic setup of a non-resonant acousto-optic modulator. A transducer generates a sound wave, at which a light beam is partially diffracted. The diffraction angle is exaggerated. ^[3]

   The acoustic wave may be absorbed at the other end of the crystal. Such a traveling-wave geometry makes it possible to achieve a broad modulation bandwidth of many megahertz. Other devices are resonant for the sound wave, exploiting the strong reflection of the acoustic wave at the other end of the crystal. The resonant enhancement can greatly increase the modulation strength (or decrease the required acoustic power), but reduces the modulation bandwidth.

Common materials for acousto-optic devices are tellurium dioxide (TeO₂), crystalline quartz, and fused silica. There are manifold criteria for the choice of the material, including the elasto-optic coefficients, the transparency range, the optical damage threshold, and required size. One may also use different kinds of acoustic waves. Most common is the use of longitudinal (compression) waves. These lead to the highest diffraction efficiencies, which however depend on the polarization of the optical beam. Polarization-independent operation is obtained when using acoustic shear waves (with the acoustic movement in the direction of the laser beam), which however make the diffraction less efficient.

There are also integrated-optical devices containing one or more acousto-optic modulators on a chip. This is possible, e.g., with integrated optics on lithium niobate (LiNbO₃), as this material is piezoelectric, so that a surface-acoustic wave can be generated via metallic electrodes on the chip surface. Such devices can be used in many ways, e.g. as tunable optical filters or optical switches. ^[3]

2. Electro-optic modulators

An electro-optic modulator (EOM) (or electrooptic modulator) is a device which can be used for controlling the power, phase or polarization of a laser beam with an electrical control signal. It typically contains one or two Pockels cells, and possibly additional optical elements such as polarizers. Different types of Pockels cells are shown in Figure 1 and are described more in detail in the article on Pockels cells. The principle of operation is based on the linear electro-optic effect (also called the Pockels effect), i.e., the modification of the refractive index of a nonlinear crystal by an electric field in proportion to the field strength.

Frequently used nonlinear crystal materials for EOMs are potassium di-deuterium phosphate (KD^*P = DKDP), potassium titanyl phosphate (KTP), beta-barium borate (BBO) (the latter for higher average powers and/or higher switching frequencies), also lithium niobate (LiNbO₃), lithium tantalate (LiTaO₃) and ammonium dihydrogen phosphate (NH₄H₂PO₄, ADP). In addition to these inorganic electro-optic materials, there are also special polymers for modulators. ^[4]

Figure 2: Pockels cells of various types. ^[4]

   The voltage required for inducing a phase change of π is called the half-wave voltage (V_π). For a Pockels cell, it is usually hundreds or even thousands of volts, so that a high-voltage amplifier is required. Suitable electronic circuits can switch such large voltages within a few nanoseconds, allowing the use of EOMs as fast optical switches. In other cases, a modulation with smaller voltages is sufficient, e.g. when only a small amplitude or phase modulation is required. ^[4]

 

 

 

3. Electroabsorption modulators

An electroabsorption modulator (or electro-absorption modulator) is a semiconductor device which can be used for controlling (modulating) the intensity of a laser beam via an electric voltage (→ optical modulators). Its principle of operation is based on the Franz–Keldysh effec^[5,6], i.e., a change in the absorption spectrum caused by an applied electric field, which changes the bandgap energy (thus the photon energy of an absorption edge) but usually does not involve the excitation of carriers by the electric field. ^[7]

Most electroabsorption modulators are made in the form of a waveguide with electrodes for applying an electric field in a direction perpendicular to the modulated light beam. For achieving a high extinction ratio, one usually exploits the quantum-confined Stark effect in a quantum well structure.

Compared with electro-optic modulators, electroabsorption modulators can operate with much lower voltages (a few volts instead of hundreds of thousands of volts). They can be operated at very high speed; a modulation bandwidth of tens of gigahertz can be achieved, which makes these devices useful for optical fiber communications. A convenient feature is that an electroabsorption modulator can be integrated with a distributed feedback laser diode on a single chip to form a data transmitter in the form of a photonic integrated circuit. Compared with direct modulation of the laser diode, a higher bandwidth and reduced chirp can be obtained.

4.Interferometric Modulator of Mach-Zehnder

An interferometer is an optical device which utilizes the effect of interference. Typically, it starts with some input beam, splits it into two separate beams with some kind of beam splitter (a partially transmissive mirror), possibly exposes some of these beams to some external influences (e.g. some length changes or refractive index changes in a transparent medium), and recombines the beams on another beam splitter. The power or the spatial shape of the resulting beam can then be used e.g. for a measurement.

The Mach–Zehnder interferometer was developed by the physicists Ludwig Mach and Ludwig Zehnder. As shown in Figure 3, it uses two separate beam splitters (BS) to split and recombine the beams, and has two outputs, which can e.g. be sent to photodetectors. The optical path lengths in the two arms may be nearly identical (as in the figure), or may be different (e.g. with an extra delay line). The distribution of optical powers at the two outputs depends on the precise difference in optical arm lengths and on the wavelength (or optical frequency). ^[8]

Figure 3: Mach–Zehnder interferometer.^[8]

   If the interferometer is well aligned, the path length difference can be adjusted (e.g. by slightly moving one of the mirrors) so that for a particular optical frequency the total power goes into one of the outputs. For misaligned beams (e.g. with one mirror being slightly tilted), there will be some fringe patterns in both outputs, and variations of the path length difference affect mainly the shapes of these interference patterns, whereas the distribution of total powers on the outputs may not change very much. ^[8]

III.CONCLUSION

   In a nutshell, there are four conclusions that i get according to the references that i have read about optical modulators. First, Acousto-optic modulators are based on the acousto-optic effect. They are used for switching or continuously adjusting the amplitude of a laser beam, for shifting its optical frequency, or its spatial direction.; second, Electro-optic modulators exploit the electro-optic effect in a Pockels cell. They can be used for modifying the polarization, phase or power of a beam, or for pulse picking in the context of ultrashort pulse amplifiers.; third, Electroabsorption modulators are intensity modulators, used e.g. for data transmitters in optical fiber communications.; fourth, Interferometric modulators, e.g. Mach–Zehnder modulators, are often realized in photonic integrated circuits for optical data transmission.

REFERENCES

[1]Optical modulator, accessed on 9 october 2016, https://www.rp-photonics.com/optical_modulators.html

[2]optical modulator, accessed on 10 october 2016, https://en.wikipedia.org/wiki/Optical_modulator

[3]Acousto-optic modulator, accessed on 10 october 2016, https://www.rp-photonics.com/acousto_optic_modulators.html

[4]Electro-optic modulator, accessed on 10 october 2016, https://www.rp-photonics.com/electro_optic_modulators.html

[5] L. V. Keldysh, “Behaviour of non-metallic crystals in strong electric fields”, J. Exp. Theor. Phys. (USSR) 33, 994 (1957); translation: Sov. Phys. JETP 6, 763 (1958)

[6]W. Franz, “Einfluß eines elektrischen Feldes auf eine optische Absorptionskante”, Z. Naturforsch., Teil A 13, 484 (1958).

[7]Electroabsorption modulators, accessed on 10 october 2016, https://www.rp-photonics.com/electroabsorption_modulators.html

[8]Interferometers, accessed on 11 october 2016, https://www.rp-photonics.com/interferometers.html

Pembahasan kali ini akan dijelaskan tentang pembuatan jaringan topologi sederhana menggunakan mininet:

1. Buatlah Topologi jaringan sederhana seperti gambar dibawah ini:
2. Setting  bandwitdth di masing-masing link switch sebesar pada link 1 sebesar 70Mbit, link 2 sebesar 100Mbit, link 3 sebesar 80 Mbit seperti gambar dibawah ini:
3. lalu setelah memasukan bandwith, klik edit lalu preference. setelah itu centang CLI lalu save. Sebelum di run,  buka terminal baru untukmengaktifkan pox seperti gambar dibawah ini:
4. lalu setelah mengaktifkan pox, klik “run”. lalu lakukan “pingall” untuk melakukan pengecekan. setelah dilakukan pengecekan didapatkan bawah masing-masing host belum terhubung. agar terhubung, maka kita melakukan penyetingan pada swicth1 – swicth 4.  seetelah penyetingan lalu kembali melakukan pengecekan maka didapatkan semua telah terhubung. penjelasan tersebut dapat dilihat dari gambar-gambar dibawah ini:
5. Selanjutnya kita akan melakukan proses filtering icmp dan udp:

   hal pertama yang dilakukan adalah melakukan penghapusan flow dengan mngetikan “sh ovs-ofctl del-flows s1” lakukan hingga s4. setelah dilakukan penghapusan flow maka langkah yang kita lakukan selanjutnya adalah menambahkan rule ARP yang berguna untuk menghandle request dari host. dengan cara mengetikan “sh ovs-ofctl add-flow s1 arp,action normal” lalu ditambahkan dengan “sh ovs-ofctl add-flows s1 priority=500,ip,ip_src=10.0.0.0/24, ip_dst=10.0.0.0/24,action=normal” lakukan hal yang sama hingga s4 seperti gambar dibawah ini:

6. langkah berikutnya adalah melakukan filtering icmp pada s1 dengan mengetikan “sh ovs-ofctl add-flows s1 priority=700,ip,icmp, action=drop” lalu melakukan pengecekan dengan pingall maka didapatkan antar host tidak tersambung. seperti gambar dibawah ini:
7. sekarang melakukan filtering UDP, yang dilakukan pertama adalah menghapus flow pada s1. setelah itu lakukan lagi memasukan ARP beserta Priority seperti langkah yang sudah dijelaskan sebelumnya. lalu ketikan command filtering seperti ini. “sh ovs-ofctl add-flows s1 priority=700,ip,udp, action=drop”. bisa dilihat seperti dibawah ini:
8. setelah dilakukan pingall maka didapatkan bahwa semua host tersambung, namun jika dilakukan pengiriman dari host 3 ke host 1 maka host 1 tidak dapat menerima paket yang dikirimkan meskipun host 3 berhasil melakukan pengiriman. Gambar dibawah merupakan bukti bahwa host 3 dapat mengirim tetapi host 1 tidak menerima paket:

Sekian Penjelasan dari pembuatan topologi dengan mininet hingga melakukan filtering. jika ada masukan bisa beritahu penulis

Semoga Bermanfaat

 

Pada sesi kali ini akan dibuat sebuah program untuk membuat segitiga piramid yang terdiri dari kumpulan bintang (*). Segitiga piramid ini akan dibuat dengan menggunakan perulangan.

Dari script coding diatas, terlihat bahwa skema pengulangan menggunakan for-in

Pertama, akan ditampilkan kata bintang dengan  perintah : print (“bintang”)

Pada baris kedua, saya menggunakan perulangan dengan batasan 1-6 dengan memakai variabel i untuk hal tersebut. Kemudian menggunakan perulangan lagi pada baris selanjutnya dengan memanfaatkan variabel j , k dan l.

Pembuatan piramida segitiga menggunakan karakter spasi dan * untuk membentuk keluaran seperti berikut ini :

Pada kesempatan kali ini, saya akan sharing bagaimana cara membuat program input data mahasiswa sederhana menggunakan program python. Program berikut membuat dua class dimana terdapat satu class induk (biodata) yang menurunkan atributnya ke class turunan (mahasiswa). Di class biodata terdapat fungsi (def) _init_() dan insert() sedangkan class mahasiswa terdapat fungsi (def) zeigen()

berikut merupakan source code nya:

Penjelasan source code :

Pada class biodata terdapat fungsi (def) _init_() yang akan dijalankan pertama kali setiap pemanggilan interasi, di fungsi ini dilakukan inisialisasi awal sedangkan terdapat juga fungsi insert() digunakan untuk memberikan masukkan atau inputan namen, nim, geburt, dan addresss.

Pada class mahasiswa yang merupakan turunan dari class biodata terdapat fungsi zeigen() yang akan menampilkan atau mengeluarkan data namen, nim, geburt, dan address yang tadi telah dimasukkan sebelumnya.

mahasiswa memanggil atribut dari class mahasiswa yang merupakan class turunan dari class biodata, dimana mahasiswa dapat memanggil fungsi yang terdapat pada class mahasiswa dan class biodata.

Berikut hasil dari program:

 

semoga  bermanfaat!

Saya Muhammad Biben Triadi, merupakan seorang mahasiswa teknik telekomunikasi di Telkom University. Saya masuk IT TELKOM(saat ini bernama Telkom University) pada tahun 2013. Saya sendiri merupakan angkatan 37 dari semejak perguruan tinggi ini dibentuk. Dalam sistem akademik yang ada di Telkom University ini, mahasiswa diwajibkan melakukan kegiatan Geladi yang merupakan syarat untuk lulus. Geladi ini dilaksanakan oleh setiap mahasiswa yang minimal sudah menempuh 4 semester perkuliahan.

Pada tanggal 1 Juni 2015 saya melaksanakan geladi di PT.CAKRA MANDIRI PRATAMA yang merupakan anak perusahaan PT.PINDAD. Di sana saya belajar banyak hal yang berkaitan dengan manufacturing. Pada awalnya saya merasa asing dengan dunia ini, karena tidak berkesinambungan dengan kompetensi jurusan Telekomunikasi yang saya pelajari di Telkom University. Tapi seiring dengan berjalannya waktu, saya mulai menikmatinya. Disana saya diperkenalkan secara umum tentang dunia manufacturing, mulai dari barang mentah hingga barang jadi. Pada hari-hari awal saya melakukan factory tour, ini bertujuan untuk memilih tema apa yang akan saya jadikan sebagai tema laporan geladi saya. Akhirnya saya pun memilih bidang pembubutan untuk tema geladi saya.

Saya disana melakukan proses bubut menggunakan mesin bubut, dari mulai mesin bubut yang sederhana hingga yang canggih(CNC). Saya melakukan beberapa proses bubut untuk beberapa produk. Hal ini memberikan saya pengalaman yang luar biasa, karena ini merupakan dunia baru dan sangat berguna untuk pengalaman saya secara pribadi. Lingkungan kerja pun disana sangat membuat saya mendapat pelajaran berharga mulai dari tata krama, kedisiplinan, sikap bekerja dengan mesin sesuai prosedur dan pedoman K3, dan lain-lain.

Dari mulai kegiatan geladi yang saya memiliki beberapa pikiran dan orientasi negatif akan kegiatan geladi saya di PT.CMP hingga akhir kegiatan geladi yang berakhir dengan senyuman, karena banyak hal positif yang saya bisa dapatkan dan itu berharga bagi kehidupan saya pribadi. Hal yang saya bisa simpulkan dari pengalaman kegiatan Geladi saya adalah segala sesuatu akan terasa indah apabila kita menjalankannya dengan rasa ikhlas.

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